Voltage Regulators AN8041S Liquid crystal backlight control IC ■ Overview Unit: mm 10.1±0.3 (0.15) 9 4.2±0.3 6.5±0.3 16 (0° to 10°) 0.3 1 8 1.5±0.2 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. ■ Features 0.1±0.1 • 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 Seating plane Note) *: The voltage is limited to the range of 3.6 V to 17 V if used in a step-down circuit. ■ Applications VREF 2.57 V R 5 S U.V.L.O. 15 CT RT 2 14 13 Q Q 8 6 Error amp. 1 Latch Q 7 9 S.C.P. comp. 11 Error amp. 2 10 CB Out FB1 IN+1 IN−1 FB2 IN+2 IN−2 GND S Bootstrap OSC PWM comp. On/off active-high R S.C.P. Constant current source 12 Off 16 3 4 1 DTC VREF ■ Block Diagram VCC • LCD displays, digital still cameras, and PDAs 1 AN8041S Voltage Regulators ■ Pin Descriptions Pin No. Symbol 1 VREF 2 RT 3 CT Description Pin No. Symbol Description Reference voltage output pin 7 IN−1 Error amplifier 1 inverted input pin Pin for connecting oscillator 8 FB1 Error amplifier 1 output pin timing resistor 9 FB2 Error amplifier 2 output pin Pin for connecting oscillator 10 IN−2 Error amplifier 2 inverted input pin timing capacitor 11 IN+2 Error amplifier 2 noninverted input pin Grounding pin 4 DTC Dead-time control pin 12 GND 5 S.C.P. Pin for connecting the time constant 13 Out Output pin 6 IN+1 setting capacitor for short-circuit 14 CB Bootstrap output circuit protection 15 VCC Power supply voltage application pin Error amplifier 1 noninverted input pin 16 Off On/off control pin ■ Absolute Maximum Ratings Parameter Symbol Rating Unit Supply voltage VCC 35 V Off terminal application voltage VOFF 35 V VI − 0.3 to VREF V DTC terminal application voltage VDTC − 0.3 to VREF V Out terminal application voltage VOUT 35 V CB terminal application voltage VCB 35 V IO ±100 mA IO(PEAK) ±500 mA PD 143 mW Topr −30 to +85 °C Tstg −40 to +125 °C Error amplifier input voltage Out terminal constant output current Out terminal peak output current Power dissipation * Operating ambient temperature Storage temperature * * Note) *: Expect for the operating ambient temperature and storage temperature, all ratings are for Ta = 25°C. ■ Recommended Operating Range Parameter 2 Symbol Range Unit Supply voltage (when using step-down circuit) VCC 3.6 to 17 V Supply voltage (when using step-up circuit) VCC 3.6 to 34 V Oscillation frequency fOUT 5 to 500 kHz Oscillator timing resistance RT 5.1 to 30 kΩ Oscillator timing capacitance CT 100 to 10 000 pF Error amplifier input voltage VIN − 0.1 to +0.8 V Reference voltage output current IREF −1 to 0 mA Voltage Regulators AN8041S ■ Electrical Characteristics at VCC = 12 V, RT = 15 kΩ, CT = 120 pF, Ta = 25°C Parameter Symbol Conditions Min Typ Max Unit 2.483 2.57 2.647 V Reference voltage block Output voltage VREF IREF = −1 mA Input regulation with input fluctuation Line VCC = 3.6 V to 34 V 7 25 mV Load regulation Load IREF = − 0.1 mA to −1 mA 1 10 mV Output voltage temperature characteristics 1 VTC1 Ta = −30°C to +25°C ±1 % Output voltage temperature characteristics 2 VTC2 Ta = 25°C to 85°C ±1 % Output short-circuit current IOS −10 mA Circuit operation start voltage VUON 2.8 3.1 3.4 V Hysteresis width VHYS 60 140 220 mV VIO −6 6 mV IB −500 −25 nA Common-mode input voltage range VICR − 0.1 0.8 V High-level output voltage 1 VEH V Low-level output voltage 1 VEL Output sink current ISINK U.V.L.O. block Error amplifier block Input offset voltage Input bias current Output source current Open-loop gain VREF − 0.3 VREF − 0.1 0.1 0.3 V VFB = 0.9 V 8 mA ISOURCE VFB = 0.9 V −110 µA 70 dB AG Dead-time control circuit block Input current IDTC RT = 15 kΩ −14.8 −12.3 −9.8 µA Low-level input threshold voltage VDT-L Duty = 0% 0.45 0.65 V High-level input threshold voltage VDT-H Duty = 100% 1.2 1.4 V Oscillation frequency fOUT RT = 15 kΩ, CT = 120 pF 180 200 220 kHz Output duty ratio Du RDTC = 75 kΩ 45 50 55 % Low-level output voltage VOL IO = 70 mA 1.0 1.3 V High-level output voltage VOH IO = −70 mA V Frequency supply voltage characteristics fdV fOUT = 200 kHz, VCC = 3.6 V to 34 V ±3 V Frequency temperature characteristics 1 fdT1 fOUT = 200 kHz, Ta = −30°C to +25°C ±9 V Frequency temperature characteristics 2 fdT2 fOUT = 200 kHz, Ta = 25°C to 85°C ±9 V Output block VCB−2.0 VCB−1.0 3 AN8041S Voltage Regulators ■ Electrical Characteristics at VCC = 12 V, RT = 15 kΩ, CT = 120 pF, Ta = 25°C (continued) Parameter Symbol Conditions Min Typ Max Unit Bootstrap circuit block Input standby voltage VINCB ICB = −70 mA VCC−1.2 VCC−1.0 VCC− 0.8 V Oscillator block VRT 0.37 V Input threshold voltage VTHPC 0.70 0.75 0.80 V Input standby voltage VSTBY 30 120 mV Input latch voltage VIN 30 120 mV Charge current ICHG −2.76 −2.3 −1.84 µA Comparator threshold voltage VTHL 1.82 V VTH 0.8 2.0 V ICC 3.9 5.0 mA ICC(SB) 5 µA RT terminal voltage Short-circuit protection block On/off control block Threshold voltage Whole device Total consumption current Standby current ■ Terminal Equivalent Circuits Pin No. 1 Equivalent circuit VCC 1 2 VREF 100 Ω DTC S.C.P. Description I/O VREF: The reference voltage output terminal (2.57 V (allowance: ±3%)). Incorporating short-circuit protection against GND. O RT: The terminal used for connecting a timing resistor to set oscillator's frequency. Use a resistance value within the range of 5.1 kΩ to 30 kΩ. The terminal voltage is approx. 0.37 V. 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. 2 RT (≈ 0.37 V) 3 VREF To PWM input IO OSC comp. 3 2IO 4 Voltage Regulators AN8041S ■ Terminal Equivalent Circuits (continued) Pin No. 4 Equivalent circuit VREF PWM comparator input IDTC 4 CDTC RDTC RT 5 VREF ICHG Latch To U.V.L.O. 0.75 V S R Q 5 6 VREF 7 7 8 6 VREF Source current 8 Sink current 9 VREF Source current 9 Sink current Description I/O 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 kΩ. 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 kΩ. 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. I 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. I 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. O 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. O 5 AN8041S Voltage Regulators ■ Terminal Equivalent Circuits (continued) Pin No. 10 Equivalent circuit VREF 11 10 11 12 12 13 VCC 14 14 13 15 15 16 17 kΩ 16 13 kΩ 6 Internal circuit Start/Stop Description I/O 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. I 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. I 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. O 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. O VCC: Power supply application terminal. I 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. I Voltage Regulators AN8041S ■ Application Notes [1] Main characteristics PD Ta Oscillation frequency Timing capacitance 600 500 Glass epoxy board (50 × 50 × 0.8t mm3) Rth(j−a) = 263°C/W PD = 380 mW (25°C) Oscillation frequency fOUT (kHz) Power dissipation PD (mW) 518 500 400 Independent IC without a heat sink Rth( j−a) = 278°C/W PD = 360 mW (25°C) 360 300 207 200 143 100 0 0 25 50 75 85 100 125 RT = 5.1 kΩ 100 RT = 15 kΩ 10 5 100 150 1 000 10 000 Ambient temperature Ta (°C) Timing capacitance CT (pF) Oscillation frequency temperature characteristics Output duty ratio temperature characteristics VCC = 12 V 54 Output duty ratio Du (%) Oscillation frequency fOUT (kHz) VCC = 12 V 205 200 195 190 185 −50 52 50 48 46 −25 0 25 50 75 −50 100 −25 0 25 50 75 100 Ambient temperature Ta (°C) Ambient temperature Ta (°C) Internal reference voltage temperature characteristics Output duty ratio DTC terminal voltage VCC = 12 V 100 Output duty ratio Du (%) Internal reference voltage VREF (V) VCC = 12 V 2.57 2.56 2.55 2.54 2.53 −50 80 60 40 20 −25 0 25 50 Ambient temperature Ta (°C) 75 100 0.2 0.4 0.6 0.8 1.0 1.2 1.4 DTC terminal voltage (V) 7 AN8041S Voltage Regulators ■ Application Notes (continued) [2] Timing chart 1. PWM comparator operation waveform High Off terminal voltage Low 3.6 V Supply voltage (VCC) Internal reference voltage (VREF) Error amplifier 1 output (FB 1) Error amplifier 2 output (FB 2) 2.57 V Power supply on DTC terminal voltage Triangular wave (CT) 1.82 V 1.32 V 0.44 V 0.03 V High S.C.P. terminal voltage Low Out terminal waveform Soft start operation Maximum duty 2. Short-ciruit protection operation waveform Internal reference voltage 2.57 V Error amplifier output (FB1) Short-circuit protection comparator threshold level 1.82 V DTC terminal voltage 1.32 V Error amplifier output (FB2) 0.44 V Triangular wave (CT) High Out terminal waveform Low 0.75 V S.C.P. terminal voltage Short-circuit protection comparator output 0.03 V tPE High Low 8 Voltage Regulators AN8041S ■ Application Notes (continued) [3] Function descriptions 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 [kΩ], 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 Voltage Regulators ■ Application Notes (continued) [3] Function descriptions (continued) 3. Error amplifier 1 block and error amplifier 2 block (continued) DC-AC inverter VOUT IN+1 IN−1 R2 R4 Error amplifier 1 block DC-DC converter output voltage detection 6 Error amp. 1 Error amp. 2 RNF1 11 IN+2 10 IN−2 7 FB2 9 R3 CT DTC R1 L A M P 16 FB1 8 VREF Backlight control R6 VC1 VR D1 SBD C1 RNF2 CNF1 R7 R5 CNF2 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. Error amp. 2 11 IN+2 To reference voltage terminal 10 IN−2 FB terminal → High-level VIN+2 > VIN−2 9 • 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. FB terminal open Figure 3. Connection when the error amplifier 2 block is not used 10 Voltage Regulators AN8041S ■ Application Note (continued) [3] Function descriptions (continued) 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 kΩ. 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. IN+1 6 Error amp. 1 IN−1 FB1 8 10 IN−2 FB2 9 S Q R Q S.C.P. comp. Latch Q1 Error amp. 2 Output cut-off Q2 1.82 V 5 IN+2 11 VREF ICHG 7 S.C.P. CS Figure 4. Short-circuit protection circuit 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 Voltage Regulators ■ Application Notes (continued) [3] Function descriptions (continued) 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 VCTH DTC waveform VDTC tON Off On IDTC CT FB VCTL Out waveform Off PWM DTC tOFF VREF 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 kΩ, CT = 150 pF), RDTC = 75 [kΩ] 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 AN8041S ■ Application Notes (continued) [3] Function descriptions (continued) 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 VD1 VGS SBD CB D1 I1 14 CB PWM comparator CT DTC FB1 FB2 Q1 I2 13 Out VCB Q2 Figure 6. Output block and bootstrap circuit VCBH VOH Turn-off VCC − VDS(ON) [V] VCC − 0.7 [V] VCC CB terminal waveform Turn-on Out terminal waveform 0V M1 source-side waveform VOL −VF t1 t2 t3 M1 Off M1 On M1 Off 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 Voltage Regulators ■ Application Notes (continued) [3] Function descriptions (continued) 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 AN8041S ■ Application Circuit Examples • Inverter control for liquid crystal backlight 15 VCC Bootstrap OSC Constant current source 13 Out Q U.V.L.O. 8 FB1 Q S 6 IN+1 R 2.3 µA Latch Q S S.C.P. 5 L A M P 14 CB PWM comp. On/Off active-high R 2 RT 12 µA SBD 15 kΩ 120 pF 3 CT 4 DTC 1 VREF 16 0.01 µF 91 kΩ VREF 2.57 V Off In 0.1 µF 3.3 kΩ V1 0.72 V 8.2 kΩ Error amp. 1 18 kΩ 7 IN−1 3 kΩ S.C.P. 9 FB2 comp. 11 IN+2 SBD 10 IN−2 104 pF Error amp. 2 GND 12 1.82 V V1 • DC-DC converter control (step-up circuit example) SBD Out In VREF 2.57 V Off 16 On/Off active-high 15 VCC 2 RT 3 CT 4 DTC 1 VREF V1 Bootstrap OSC Constant current source 14 CB PWM comp. 13 Out R Q U.V.L.O. Q S S.C.P. 5 S 6 IN+1 Latch Q Error amp. 1 7 IN−1 9 FB2 S.C.P. comp. 11 IN+2 10 IN−2 1.82 V GND 12 R 8 FB1 Error amp. 2 V1 15