Datasheet LTC3801, LTC3801B (Analog Devices) - 6
| Hersteller | Analog Devices |
| Beschreibung | Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT |
| Seiten / Seite | 12 / 6 — OPERATIO. (Refer to the Functional Diagram). VOUT Ripple for Front Page … |
| Dateiformat / Größe | PDF / 243 Kb |
| Dokumentensprache | Englisch |
OPERATIO. (Refer to the Functional Diagram). VOUT Ripple for Front Page Circuit Using the LTC3801
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LTC3801/LTC3801B
U OPERATIO (Refer to the Functional Diagram) VOUT Ripple for Front Page Circuit Using the LTC3801 VOUT Ripple for Front Page Circuit Using the LTC3801B (with Burst Mode Operation) (Burst Mode Operation Disabled)
20mVAC/DIV 20mVAC/DIV VIN = 4.2V 5µs/DIV 3801 F01a VIN = 4.2V 5µs/DIV 3801 F01b VOUT= 2.5V VOUT= 2.5V IOUT = 100mA IOUT = 100mA
Figure 1. Output Ripple Waveforms for the Front Page Circuit
100 VOUT = 2.5V This lower frequency allows the inductor current to safely 95 discharge, thereby preventing current runaway. After the 90 VIN = 3.3V short is removed, the oscillator frequency will gradually 85 increase back to 550kHz as V 80 FB rises through 0.3V on its way back to 0.8V. 75 VIN = 4.2V 70 EFFICIENCY (%)
Overvoltage Protection
65 VIN = 6.6V VIN = 8.4V V 60 IN = 9.8V If VFB exceeds its regulation point of 0.8V by more than 55 10% for any reason, such as an output short circuit to a 50 higher voltage, the overvoltage comparator will hold the 0.1 1 10 100 1000 10000 LOAD CURRENT (mA) external P-channel MOSFET off. This comparator has a 3801 F02 typical hysteresis of 40mV.
Figure 2. LTC3801B Efficiency vs Load Current Slope Compensation and Inductor’s Peak Current Undervoltage Lockout Protection
The switch on duty cycle in normal operation is given by: To prevent operation of the external P-channel MOSFET with insufficient gate drive, an undervoltage lockout cir- V V Duty OUT + D Cycle = cuit is incorporated into the LTC3801/LTC3801B. When V + V IN D the input supply voltage drops below approximately 1.7V, where V the P-channel MOSFET and all internal circuitry other than D is the forward voltage drop of the external diode at the average inductor current. For duty cycles less than the undervoltage block itself are turned off. Input supply 40%, the inductor’s peak current is determined by: current in undervoltage is approximately 1µA. V V ITH RUN
Short-Circuit Protection
IMAX = / – . 0 7 10RSENSE If the output is shorted to ground, the frequency of the oscillator is folded back from 550kHz to approximately However, for duty cycles greater than 40%, slope com- 210kHz while maintaining the same minimum on time. pensation begins and effectively reduces the peak sn3801 3801fs 6
U OPERATIO (Refer to the Functional Diagram) VOUT Ripple for Front Page Circuit Using the LTC3801 VOUT Ripple for Front Page Circuit Using the LTC3801B (with Burst Mode Operation) (Burst Mode Operation Disabled)
20mVAC/DIV 20mVAC/DIV VIN = 4.2V 5µs/DIV 3801 F01a VIN = 4.2V 5µs/DIV 3801 F01b VOUT= 2.5V VOUT= 2.5V IOUT = 100mA IOUT = 100mA
Figure 1. Output Ripple Waveforms for the Front Page Circuit
100 VOUT = 2.5V This lower frequency allows the inductor current to safely 95 discharge, thereby preventing current runaway. After the 90 VIN = 3.3V short is removed, the oscillator frequency will gradually 85 increase back to 550kHz as V 80 FB rises through 0.3V on its way back to 0.8V. 75 VIN = 4.2V 70 EFFICIENCY (%)
Overvoltage Protection
65 VIN = 6.6V VIN = 8.4V V 60 IN = 9.8V If VFB exceeds its regulation point of 0.8V by more than 55 10% for any reason, such as an output short circuit to a 50 higher voltage, the overvoltage comparator will hold the 0.1 1 10 100 1000 10000 LOAD CURRENT (mA) external P-channel MOSFET off. This comparator has a 3801 F02 typical hysteresis of 40mV.
Figure 2. LTC3801B Efficiency vs Load Current Slope Compensation and Inductor’s Peak Current Undervoltage Lockout Protection
The switch on duty cycle in normal operation is given by: To prevent operation of the external P-channel MOSFET with insufficient gate drive, an undervoltage lockout cir- V V Duty OUT + D Cycle = cuit is incorporated into the LTC3801/LTC3801B. When V + V IN D the input supply voltage drops below approximately 1.7V, where V the P-channel MOSFET and all internal circuitry other than D is the forward voltage drop of the external diode at the average inductor current. For duty cycles less than the undervoltage block itself are turned off. Input supply 40%, the inductor’s peak current is determined by: current in undervoltage is approximately 1µA. V V ITH RUN
Short-Circuit Protection
IMAX = / – . 0 7 10RSENSE If the output is shorted to ground, the frequency of the oscillator is folded back from 550kHz to approximately However, for duty cycles greater than 40%, slope com- 210kHz while maintaining the same minimum on time. pensation begins and effectively reduces the peak sn3801 3801fs 6