Text preview for : AN8041S.pdf part of Panasonic AN8041S LCD display backlight control IC
Back to : AN8041S.pdf | Home
Voltage Regulators
AN8041S
Liquid crystal backlight control IC
s Overview
The AN8041S is an inverter control IC for liquid crystal backlight using PWM method. The output voltage of DC-DC converter and the current of cathode-ray tube can be controlled by using two error amplifiers, so that the system is designed easily. Since the n-channel MOSFET can be directly driven, it is possible to construct a highly effective power supply.
10.1±0.3 16 9
Unit: mm
4.2±0.3 6.5±0.3
(0.15)
0.3 1 8
1.5±0.2
(0° to 10°)
s Features
· Operating supply voltage: 3.6 V to 34 V * 1.27 0.40±0.25 (0.605) · Totem pole output circuit: Output current of ±500 mA Seating plane · Built-in bootstrap circuit · N-channel power MOSFET can be directly driven SOP016-P-0225A · Built-in two error amplifier circuits allow both the voltage and current control · Incorporating on/off functions (active-high control input, standby mode current is 5 µA or less) · Built-in timer latch short-circuit protection circuit · Maximum oscillation frequency: 500 kHz
0.1±0.1
Seating plane
Note) *: The voltage is limited to the range of 3.6 V to 17 V if used in a step-down circuit.
s Applications
· LCD displays, digital still cameras, and PDAs
VREF
DTC
s Block Diagram
1
4
3
2
VREF 2.57 V Off 16 On/off active-high R S R S.C.P. 5 S Q Q
Constant current source
OSC
Bootstrap
15
VCC
CT
RT
14 13 8 6
PWM comp.
CB Out FB1 IN+1 IN-1 FB2 IN+2 IN-2
U.V.L.O.
Latch Q S.C.P. comp.
Error amp. 1
7 9 11
Error amp. 2
GND 12
10
1
AN8041S
s Pin Descriptions
Pin No. Symbol 1 2 VREF RT Description Reference voltage output pin Pin for connecting oscillator timing resistor 3 CT Pin for connecting oscillator timing capacitor 4 5 DTC S.C.P. Dead-time control pin Pin for connecting the time constant setting capacitor for short-circuit protection 6 IN+1 Error amplifier 1 noninverted input pin Pin No. Symbol 7 8 9 10 11 12 13 14 15 16 IN-1 FB1 FB2 IN-2 IN+2 GND Out CB VCC Off
Voltage Regulators
Description Error amplifier 1 inverted input pin Error amplifier 1 output pin Error amplifier 2 output pin Error amplifier 2 inverted input pin Error amplifier 2 noninverted input pin Grounding pin Output pin Bootstrap output circuit Power supply voltage application pin On/off control pin
s Absolute Maximum Ratings
Parameter Supply voltage Off terminal application voltage Error amplifier input voltage DTC terminal application voltage Out terminal application voltage CB terminal application voltage Out terminal constant output current Out terminal peak output current Power dissipation
* *
Symbol VCC VOFF VI VDTC VOUT VCB IO IO(PEAK) PD Topr Tstg
Rating 35 35 - 0.3 to VREF - 0.3 to VREF 35 35 ±100 ±500 143 -30 to +85 -40 to +125
Unit V V V V V V mA mA mW °C °C
Operating ambient temperature Storage temperature
*
Note) *: Expect for the operating ambient temperature and storage temperature, all ratings are for Ta = 25°C.
s Recommended Operating Range
Parameter Supply voltage (when using step-down circuit) Supply voltage (when using step-up circuit) Oscillation frequency Oscillator timing resistance Oscillator timing capacitance Error amplifier input voltage Reference voltage output current Symbol VCC VCC fOUT RT CT VIN IREF Range 3.6 to 17 3.6 to 34 5 to 500 5.1 to 30 100 to 10 000 - 0.1 to +0.8 -1 to 0 Unit V V kHz k pF V mA
2
Voltage Regulators
s Electrical Characteristics at VCC = 12 V, RT = 15 k, CT = 120 pF, Ta = 25°C
Parameter Reference voltage block Output voltage Input regulation with input fluctuation Load regulation Output voltage temperature characteristics 1 Output voltage temperature characteristics 2 Output short-circuit current U.V.L.O. block Circuit operation start voltage Hysteresis width Error amplifier block Input offset voltage Input bias current Common-mode input voltage range High-level output voltage 1 Low-level output voltage 1 Output sink current Output source current Open-loop gain Dead-time control circuit block Input current Low-level input threshold voltage High-level input threshold voltage Output block Oscillation frequency Output duty ratio Low-level output voltage High-level output voltage Frequency supply voltage characteristics Frequency temperature characteristics 1 Frequency temperature characteristics 2 fOUT Du VOL VOH fdV fdT1 fdT2 RT = 15 k, CT = 120 pF RDTC = 75 k IO = 70 mA IO = -70 mA fOUT = 200 kHz, VCC = 3.6 V to 34 V fOUT = 200 kHz, Ta = -30°C to +25°C fOUT = 200 kHz, Ta = 25°C to 85°C 180 45 200 50 1.0 ±3 ±9 ±9 IDTC VDT-L VDT-H RT = 15 k Duty = 0% Duty = 100% -14.8 1.2 -12.3 0.45 1.4 VIO IB VICR VEH VEL ISINK VFB = 0.9 V ISOURCE VFB = 0.9 V AG -6 -500 - 0.1 -25 VUON VHYS 2.8 60 3.1 140 VREF Line Load VTC1 VTC2 IOS IREF = -1 mA VCC = 3.6 V to 34 V IREF = - 0.1 mA to -1 mA Ta = -30°C to +25°C Ta = 25°C to 85°C 2.483 2.57 7 1 ±1 ±1 -10 Symbol Conditions Min Typ
AN8041S
Max
Unit
2.647 25 10
V mV mV % % mA
3.4 220
V mV
6 0.8 0.3 -9.8 0.65
mV nA V V V mA µA dB µA V V
VREF - 0.3 VREF - 0.1 0.1 8 -110 70
220 55 1.3
kHz % V V V V V
VCB-2.0 VCB-1.0
3
AN8041S
Voltage Regulators
s Electrical Characteristics at VCC = 12 V, RT = 15 k, CT = 120 pF, Ta = 25°C (continued)
Parameter Bootstrap circuit block Input standby voltage Oscillator block RT terminal voltage Short-circuit protection block Input threshold voltage Input standby voltage Input latch voltage Charge current Comparator threshold voltage On/off control block Threshold voltage Whole device Total consumption current Standby current ICC ICC(SB) 3.9 5.0 5 mA µA VTH 0.8 2.0 V VTHPC VSTBY VIN ICHG VTHL 0.70 -2.76 0.75 30 30 -2.3 1.82 0.80 120 120 -1.84 V mV mV µA V VRT 0.37 V VINCB ICB = -70 mA VCC-1.2 VCC-1.0 VCC- 0.8 V Symbol Conditions Min Typ Max Unit
s Terminal Equivalent Circuits
Pin No. 1 VCC Equivalent circuit Description VREF: The reference voltage output terminal (2.57 V (allowance: ±3%)). Incorporating short-circuit protection against GND. RT: The terminal used for connecting a timing resistor to set oscillator's frequency. Use a resistance value within the range of 5.1 k to 30 k. The terminal voltage is approx. 0.37 V. I/O O
1 2
VREF
100 DTC S.C.P. 2 RT ( 0.37 V)
3
VREF To PWM input OSC comp. IO 3 2IO
CT: The terminal used for connecting a timing capacitor to set oscillator's frequency. Use a capacitance value within the range of 100 pF to 10 000 pF. For frequency setting method, refer to the " Application Notes, [3] Function descriptions " section. Use an oscillation frequency in the range of 5 kHz to 500 kHz.
4
Voltage Regulators
s Terminal Equivalent Circuits (continued)
Pin No. 4 VREF PWM comparator input IDTC RDTC Equivalent circuit Description
AN8041S
I/O
4 CDTC RT 5
VREF ICHG To U.V.L.O. 0.75 V
DTC: The terminal for connecting a resistor and capacitor to set the dead-time and soft start period of PWM output. Input current IDTC is determined by the timing resistor RT , so that dispersion and fluctuation with temperature are suppressed. It is approx. -12.3 µA when RT = 15 k. VRT 1 IDTC = × [A] RT 2 S.C.P.: The terminal for connecting a capacitor to set the time constant of soft start and timer latch shortcircuit protection circuit. Use a capacitance value in the range of more than 1 000 pF. The charge current ICHG is determined by the timing resistor RT , so that dispersion and fluctuation with temperature are suppressed. It is approx. -1.3 µA when RT = 15 k. VRT 1 ICHG = × [A] RT 11 IN+1: The noninverted input terminal of the error amplifier 1. For common-mode input, use in the range of - 0.1 V to +0.8 V. IN-1: The inverted input terminal of the error amplifier 1. For common-mode input, use in the range of - 0.1 V to +0.8 V. FB1: The output terminal of the error amplifier 1. Source current : approx. -120 µA Sink current : approx. 8 mA Correct the frequency characteristics of the gain and the phase by connecting a resistor and a capacitor between this terminal and IN-1 terminal. FB2: The output terminal of the error amplifier 2. Source current : approx. -120 µA Sink current : approx. 8 mA Correct the frequency characteristics of the gain and the phase by connecting a resistor and a capacitor between this terminal and IN-2 terminal.
Latch S R Q
5
6
VREF
I
7
I
7
6
8
VREF Source current 8 Sink current
O
9
VREF Source current 9 Sink current
O
5
AN8041S
s Terminal Equivalent Circuits (continued)
Pin No. 10
VREF
Voltage Regulators
Equivalent circuit
Description IN-2: The inverted input terminal of the error amplifier 2. For common-mode input, use in the range of - 0.1 V to +0.8 V. IN+2: The noninverted input terminal of the error amplifier 2. For common-mode input, use in the range of - 0.1 V to +0.8 V. GND: Grounding terminal. Out: Totem pole type output terminal. A constant output current of ±100 mA and a peak output current of ±1 A can be obtained.
I/O I
11
I
10
11
12
12
13 VCC
O
14
14 13
CB: Bootstrap output terminal. When using step-down circuit, connect the capacitor for boost between this terminal and the n-channel MOSFET source side of the switching device. When using step-up circuit, short circuit this terminal with VCC terminal. VCC: Power supply application terminal. Off: On/off control terminal. High-level input: normal operation (VOFF > 2.0 V) Low-level input: standby condition (VOFF < 0.8 V) The total consumption current can be suppressed to 10 µA or less.
O
15
15
I
16
Internal circuit Start/Stop
I
17 k 16 13 k
6
Voltage Regulators
s Application Notes
[1] Main characteristics PD Ta
600 518 500 Glass epoxy board (50 × 50 × 0.8t mm3) Rth(j-a) = 263°C/W PD = 380 mW (25°C) Independent IC without a heat sink Rth( j-a) = 278°C/W PD = 360 mW (25°C)
AN8041S
Oscillation frequency Timing capacitance
500
Oscillation frequency fOUT (kHz)
Power dissipation PD (mW)
400 360 300 207 200 143 100
RT = 5.1 k 100
RT = 15 k
10
0 0 25 50 75 85 100 125 150
5 100
1 000
10 000
Ambient temperature Ta (°C)
Timing capacitance CT (pF)
Oscillation frequency temperature characteristics
VCC = 12 V
Output duty ratio temperature characteristics
VCC = 12 V 54
Oscillation frequency fOUT (kHz)
205
Output duty ratio Du (%)
-25
200
52
195
50
190
48
185
46
-50
0
25
50
75
100
-50
-25
0
25
50
75
100
Ambient temperature Ta (°C)
Ambient temperature Ta (°C)
Internal reference voltage temperature characteristics
VCC = 12 V
Output duty ratio DTC terminal voltage
VCC = 12 V 100
Internal reference voltage VREF (V)
2.57
2.56
Output duty ratio Du (%)
-25 0 25 50 75 100
80
2.55
60
2.54
40
2.53
20
-50
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Ambient temperature Ta (°C)
DTC terminal voltage (V)
7
AN8041S
s Application Notes (continued)
[2] Timing chart 1. PWM comparator operation waveform
Voltage Regulators
Off terminal voltage Supply voltage (VCC) Error amplifier 1 output (FB 1) Error amplifier 2 output (FB 2)
High Low 3.6 V
Internal reference voltage (VREF)
2.57 V 1.82 V 1.32 V 0.44 V 0.03 V High S.C.P. terminal voltage Out terminal waveform Low
Power supply on
DTC terminal voltage Triangular wave (CT)
Soft start operation
Maximum duty
2. Short-ciruit protection operation waveform
Internal reference voltage Error amplifier output (FB1) Short-circuit protection comparator threshold level DTC terminal voltage Error amplifier output (FB2) Triangular wave (CT) Out terminal waveform
2.57 V
1.82 V
1.32 V 0.44 V
High Low 0.75 V
S.C.P. terminal voltage Short-circuit protection comparator output tPE
0.03 V High Low
8
Voltage Regulators
s Application Notes (continued)
[3] Function descriptions
AN8041S
1. Reference voltage block This block is composed of the band gap circuit, and outputs the temperature-compensated 2.57 V reference voltage to the VREF terminal (pin 16). The reference voltage is stabilized when the supply voltage is 3.6 V or higher, and used as the operating power supply for the IC inside. It is possible to take out a load current of up to -1 mA. 2. Triangular wave oscillation block (OSC) The triangular wave which swings from the upper limit value VOSCH of approximately 1.32 V to the lowest limit value VOSCL of approximately 0.44 V will be generated by connecting a timing capacitor CT and a resistor RT to the CT terminal (pin 2) and RT terminal (pin 3) respectively. The oscillation frequency can be arbitrarily decided by the value of timing capacitor CT and resistor RT connected externally. The oscillation frequency fOSC is obtained by the following calculations: VCTH = 1.32 V typ. 1 IO fOSC = = t1 + t 2 2 × CT × (VCTH - VCHL) 1.7 × VRT 1.7 × 0.37 IO = = RT RT VCTL = 0.44 V typ. Since VCTH - VCTL = 0.88 V, t1 t2 1 therefore fOSC [Hz] Charging Discharging 2.80 × CT × RT Example) When CT = 100 [pF], RT = 15 [k], fOSC 238 [kHz].
T Figure 1. Triangular wave oscillation waveform
It is possible to use the circuit in the recommended operating range of 5 kHz to 500 kHz of the oscillation frequency. In addition, when the oscillation frequency becomes high, overshoot and undershoot are generated due to the operation delay of the triangular oscillation comparator. Care should be taken because the actual measurement values deviate from the above calculation values. In the case of this IC, the output source current of S.C.P. terminal and DTC terminal are set by the timing resistor RT externally attached to RT terminal. For this reason, the AN8041S can not be used as a slave IC when multiple ICs are synchronously operating in parallel. 3. Error amplifier 1 block and error amplifier 2 block DC-DC output voltage and a detected lamp current of back-light are amplified through the PNP transistor input type error amplifier, and the amplified signal are inputted to PWM comparator. Figure 2 shows the connection method of the error amplifier when the backlight inverter is controlled. Select the connection of error amplifier 1 block or 2 block arbitrarily. The common-mode input range is from - 0.1 V to +0.8 V. The voltage which is resistor-dividing of the reference voltage is given to the noninverted input. Also, any desired gain setting and phase compensation can be obtained by connecting the feedback resistor and capacitor from the error amplifier output terminals (pin 8 and pin 9) to the inverted input terminals (pin 7 and pin 10). The overshoot at operation start due to feedback delay can be suppressed by providing the large output source current (110 µA) and the large output sink current (8 mA). The output voltage VOUT and the detection voltage of the lamp current VC1 are given from the following calculation: R4 VIN+ = VREF × R3 + R4 R1 + R2 VOUT = VIN+1 × R2 R5 + VR + R6 VC1 = VIN+2 × R5 + VR
9
AN8041S
s Application Notes (continued)
[3] Function descriptions (continued) 3. Error amplifier 1 block and error amplifier 2 block (continued)
Voltage Regulators
VOUT VREF R1 R3 IN+1 IN-1 16 6 7
FB1 8 FB2 9 CT DTC
DC-AC inverter L A M P Backlight control R6 VC1 D1 SBD C1
Error amp. 1
Error amp. 2
11 IN+2 10 IN-2
R2
R4
RNF1 CNF1
RNF2 CNF2
VR R5
R7
Error amplifier 1 block DC-DC converter output voltage detection
Error amplifier 2 block Backlight lamp current detection
Figure 2. Connection method of the error amplifier 1 and 2
The control modes of backlight are described below: 1) Power-on mode When the power supply is turned on, the DC-DC converter which is connected to the error amplifier 1 block starts the control. The output voltage VOUT which has been set by the equation in the previous page is reached, and the high voltage of several kV is generated in the lamp through the DC-AC inverter, and the backlight is lighted up. During this period, since the lamp current does not flow in the error amplifier 2 block, the error amplifier output (FB2) becomes high-level, so that its control does not work. 2) Normal control mode When the backlight is turned on, discharging starts and the current starts to flow in the resistor R7. When the voltage VC1 rectified by diode D1, and capacitor C1 reaches the voltage set by resistors R5, R6, and volume control VR ; The control function is switched over from the error amplifier 1 block to the error amplifier 2 block. The output voltage of the DC-DC converter VOUT decreases to a voltage lower than the set voltage, and the lamp voltage is maintained at several hundred volts. 3) Light-regulation operation mode For the light regulation of the backlight, the "voltage light-regulation" method is used, and the light is regulated by the input voltage of the inverter. By adding volume VR to the inverted input terminal of error amplifier 2 block to make the detection voltage VC1 variable, the input voltage of the inverter is regulated so as to make the lamp current variable for light regulation. Also, the addition of the volume to the noninverted input side of the error amplifier makes the light regulation possible. · Usage notes When this IC is used to control the DC-DC converter, one of two error amplifiers is not used. Connection should be made so that the FB terminal is fixed to high-level as shown in figure 3.
Error amp. 2 11 IN+2 10 IN-2 FB terminal High-level VIN+2 > VIN-2 FB terminal open Figure 3. Connection when the error amplifier 2 block is not used To reference voltage terminal
10
9
Voltage Regulators
s Application Note (continued)
[3] Function descriptions (continued)
AN8041S
4. Timer latch short-circuit protection circuit When the short-circuit or overload of the power supply output continues for a certain period, this circuit prevents the parts such as external main switch device, flywheel diode, the choke coil from destruction or deterioration. The short-circuit protection circuit is shown in figure 4. The timer latch short-circuit protection circuit detects the output level of the error amplifier 1 and 2 blocks. When either the DC-DC converter output voltage or the lamp current detection voltage is stable, the output of that error amplifier is stabilized and the short-circuit protection comparator also maintains balance. When the load conditions are suddenly changed, and both of the outputs of the error amplifier 1 block and 2 block (FB1, FB2) become 1.82 V or higher, the short-circuit protection comparator outputs low-level and cut off the transistor Q1, thereby the external capacitor CS of the S.C.P. terminal (pin 5) starts charging with current ICHG given by the following equation: tPE VPE = VSTBY + ICHG × [V] CS tPE 0.75 V = 0.03 V + ICHG × CS tPE CS = ICHG × [F] 0.72 ICHG is constant current which is determined by the timing resistor RT of the oscillator. It becomes approximately 2.3 µA when RT = 15 k. VRT × 1 ICHG = [A] RT × 11 When the external capacitor CS is charged to approximately 0.75 V, the latch circuit is set to fix the totem pole output terminal to low-level and sets the dead-time to 100%. When the latch circuit is set, the S.C.P. terminal voltage is discharged to approximately 30 mV. However, once the latch circuit is set, it is not reset unless the power supply is turned off.
Error amp. 1 ICHG S.C.P. comp. Error amp. 2 1.82 V Q1 Q2 S Q R Q Latch
IN+1
6 7
VREF Output cut-off
IN-1 FB1 8 IN+2
11
S.C.P.
5
CS Figure 4. Short-circuit protection circuit
10 IN-2 FB2 9
5. Low input voltage malfunction prevention circuit (U.V.L.O.) When the supply voltage is dropped under the transient condition such as power-on or operation stop, this circuit protects the system from destruction or deterioration due to the malfunction of the control circuit. This circuit detects the internal reference voltage which varies according to the supply voltage level. During the period from the time when the supply voltage starts to rise and to the time when it reaches 3.1 V, it keeps the deadtime of the Out terminal (pin 13) to 100% and maintains the DTC terminal (pin 4) and the S.C.P. terminal (pin 5) at low-level. When the supply voltage falls, it holds the hysteresis width of 140 mV and operates at a voltage under 2.96 V. 11
AN8041S
s Application Notes (continued)
[3] Function descriptions (continued)
Voltage Regulators
6. Remote circuit The IC control function can be turned on or off by the external control. When the voltage of Off terminal (pin 16) is set under approximately 0.8 V, the internal reference voltage falls to stop the IC control function, and decrease the circuit current to a value under 5 µA, When the voltage of Off terminal is set at a value higher than approximately 2.0 V, the internal reference voltage rises, and starts the control operation. 7. PWM comparator block The PWM comparator controls the on-period of the output pulse according to the input voltage. While the voltage of triangular wave of the CT terminal (pin 3) is lower than any one of the output of the error amplifier 1 and 2 block (pin 8 and pin 9) and the voltage of the DTC terminal (pin 4), it sets the Out terminal (pin 13) to highlevel so that the switching device (n-channel MOSFET) turns on . The dead-time is set by regulating the DTC terminal voltage VDTC as shown in figure 5. The DTC terminal is of a constant current output using the resistor RT, so that the VDTC is regulated by connecting the external resistor RDTC between the DTC terminal and GND terminal. At the oscillation frequency fOSC of 200 kHz, the output duty ratio becomes 0% when VDTC = 0.44 V typical, and 100% when VDTC = 1.32 V typical. However, pay attention to the peak value VCTH and the trough value VCTL of the triangular wave because their overshoot and undershoot amount differ depending on the oscillation frequency. CT waveform DTC waveform tOFF Out waveform Off On Off tON VCTH VDTC VCTL DTC VREF IDTC CT FB PWM
RDTC
CDTC
Figure 5. Setting the dead-time
The output duty ratio Du and the DTC terminal voltage VDTC are given in the following equation: tON VRT 1 × 100 [%] IDTC = × Du = [A] RT 2 tON + tOFF VDTC = IDTC × RDTC VDTC - VCTL = × 100 [%] VCTH - VCTL RDTC 1 = VRT × × [A] 2 RT Example) When fOSC = 200 [kHz] (RT = 15 k, CT = 150 pF), RDTC = 75 [k] VCTH 1.32 [V], VCTL 0.44 [V], VDTC 0.37 [V] Therefore IDTC 12.3 [µA] VDTC 0.925 [V] Du 55.1 [%] In addition, the operation delay of the PWM comparator, the deviation of the peak and trough triangular oscillation value may cause the deviation of the actual measurements value from the theoretical value. So, regulation on IC-mounted PCB should be required. By adding the external resistor RDTC and capacitor CDTC , the soft start function can be installed, which gradually broadens the on-period of the output pulse at the time of the power supply operation start. The soft start operation prevents the overshoot of DC-DC comparator output.
12
Voltage Regulators
s Application Notes (continued)
[3] Function descriptions (continued)
AN8041S
15 VCC
8. Output block, bootstrap circuit In the case of the step-down type DC-DC converter control, the bootstrap circuit is required if n-channel MOSFET is used as the switching device. The bootstrap circuit is used for keeping the voltage between the gate and the source higher than the gate threshold voltage of n-channel MOSFET by increasing the high-level of the Out terminal (pin 13) to a level higher than VCC when turning on the n-channel MOSFET. The output block including the external circuit and the bootstrap circuit are shown in figure 6, and the operation waveform in figure 7. VS M1 VOUT VGS SBD CB D1 PWM comparator CT DTC FB1 FB2 Q1 I1 14 CB I2 13 Out Q2 VCB
VD1
Figure 6. Output block and bootstrap circuit
VCBH VOH Turn-off VCC CB terminal waveform Turn-on Out terminal waveform 0V M1 source-side waveform VOL -VF t1 M1 Off t2 M1 On t3 M1 Off VCC - VDS(ON) [V] VCC - 0.7 [V]
Figure 7. Bootstrap circuit operation waveform
The bootstrap circuit operation is described below. 1) N-channel MOSFET (M1) off time: t1 While the M1 is off, energy is being supplied from the schottky barrier diode (SBD) to the choke coil, and the M1 source side voltage VS is fixed to -VF . The capacitor for boost CB is charged from the VCC terminal (pin 15) through the diode inside the IC (D1). The CB terminal voltage (pin 14) VCB is given by the following equation: VS = -VF VCB = VCC -VD1 VF : forward voltage of SBD VD1 : forward voltage of D1 Therefore, the charged voltage of boost CB is given by the following equation: VCB-VS = VCC -VD1+VF 13
AN8041S
s Application Notes (continued)
[3] Function descriptions (continued)
Voltage Regulators
8. Output block, bootstrap circuit (continued) 2) N-channel MOSFET (M1) turn-on time: t2 When the PWM comparator output reverses, the Out terminal (pin 13) is switched over to high-level. The Out terminal voltage VO rises toward the CB terminal voltage. VO = VCB-VCE (sat) At that time, M1 voltage between the gate and source becomes: VGS = VO+VF When the Out terminal voltage VO rises to the gate threshold voltage, the M1 is turned on. The M1 sourceside voltage after the turn-on rises to the value expressed in the following equation: VS = VCC -VDS(ON) Since the bootstrap capacitor CB is connected between the M1 source-side and the CB terminal, the CB terminal voltage is capacitance-coupled, and rises according to the M1 source-side voltage. It is expressed in the following equation: VCB = VS +VCC -VD1+VF = 2 × VCC -VD1+VDS(ON)+VF 3) N-channel MOSFET (M1) turn-off time: t3 The Out terminal voltage drops to the saturation voltage of the transistor Q1 and it is turned off. The M1 source side voltage decreases to -VF , and in the same way the CB terminal voltage is capacitancecoupled, and drops to VCC -VD1 volt, and returns to the condition described in a). · Bootstrap circuit usage notes (1) Operating supply voltage range when the step-down circuit is used When the step-down circuit is used for the DC-DC converter control : As described in the above, when the n-channel MOSFET of the switching device turns on, the voltage of CB terminal (pin 14) rises to the voltage about two times higher than the VCC . Since the allowable applied voltage for the CB terminal is 35 V, use the boost circuit at an operating supply voltage of 3.6 V or more. VCB = 2 × VCC - VD1 - VDS(ON) + VF < 35 [V] 35 + VD1 + VDS(ON) - VF VCC < [V] 2 < 17 [V] (2) Value setting for bootstrap capacitor The bootstrap capacitor is capacitors-coupled with the n-channel MOSFET source-side at its turn-on time to increase the CB terminal voltage over the VCC . At this time, the bootstrap capacitor is discharged by the n-channel MOSFET gate drive current. If the capacitance value of the bootstrap capacitor is set at too low value, it causes the efficiency decrease due to increase in switching loss. Therefore, set the capacitance at a sufficiently high value compared with the n-channel MOSFET gate input capacitance. CB >> Ciss Study with the actual mounting board and set the optimum value. (3) CB terminal connection when the booster circuit is used In the case of using the step-up type DC-DC converter control, the bootstrap circuit is not required since the n-channel MOSFET source side is grounded. Therefore, use it by short-circuiting the CB terminal (pin 14) to the VCC terminal (pin 15). For that reason, the operating supply voltage range is 3.6 V to 34 V in the case of using the step-up circuit type.
14
Voltage Regulators
s Application Circuit Examples
· Inverter control for liquid crystal backlight
3.3 k V1 0.72 V 8.2 k 0.1 µF 91 k 0.01 µF 120 pF 15 k In SBD
AN8041S
1 VREF
4 DTC
12 µA
VREF 2.57 V Off 16
Constant current source
OSC
Bootstrap 14 CB
15 VCC
3 CT
2 RT
On/Off active-high
R S R
PWM comp. 13 Out
L A M P
U.V.L.O.
Q Q 2.3 µA 8 FB1 6 IN+1 7 IN-1 9 FB2 11 IN+2 10 IN-2 SBD Error amp. 2 V1 18 k 3 k
S.C.P. 5
S
Latch Q
Error amp. 1
S.C.P.
comp.
104 pF
1.82 V
· DC-DC converter control (step-up circuit example)
In V1 1 VREF 4 DTC 15 VCC 3 CT 2 RT
GND 12
SBD Out
VREF 2.57 V 16
Constant current source
OSC
Bootstrap 14 CB
Off
On/Off active-high
R Q U.V.L.O. Q S Latch Q
PWM comp. 13 Out
8 FB1 6 IN+1 Error amp. 1 S.C.P. comp. 1.82 V Error amp. 2 V1 7 IN-1 9 FB2 11 IN+2 10 IN-2
R S.C.P. 5 S
GND 12
15