Datasheet LTC3701 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | 2-Phase, Low Input Voltage, Dual Step-Down DC/DC Controller |
| Pages / Page | 20 / 8 — OPERATIO. (Refer to Functional Diagram). Dropout Operation. Slope … |
| File Format / Size | PDF / 261 Kb |
| Document Language | English |
OPERATIO. (Refer to Functional Diagram). Dropout Operation. Slope Compensation and Peak Inductor Current. Undervoltage Lockout
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LTC3701
U OPERATIO (Refer to Functional Diagram) Dropout Operation Slope Compensation and Peak Inductor Current
When the input supply voltage decreases towards the The inductor’s peak current is determined by: output voltage, the rate of change of the inductor current during the ON cycle decreases. This reduction means that V 0 7V I ITH RUN PK = / – . the external P-channel MOSFET will remain on for more 10 • RSENSE than one oscillator cycle if the inductor current has not when the LTC3701 is operating below 20% duty cycle. ramped up to the threshold set by EAMP on the ITH/RUN However, once the duty cycle exceeds 20%, slope pin. Further reduction in input supply voltage will eventu- ally cause the P-channel MOSFET to be turned on 100%, compensation begins and effectively reduces the peak inductor current. The amount of reduction is given by the i.e., DC. The output voltage will then be determined by the curve in Figure 2. input voltage minus the voltage drop across the MOSFET, the sense resistor and the inductor. 110 100
Undervoltage Lockout
90 80 To prevent operation of the P-channel MOSFET below safe 70 input voltage levels, an undervoltage lockout is incorpo- (%) 60 MAX rated into the LTC3701. When the input supply voltage 50 drops below 2V, the P-channel MOSFET and all circuitry SF = I/I 40 are turned off except the undervoltage block, which draws 30 20 only several microamperes. 10 0
Short-Circuit Protection
0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) When an output is shorted to ground (VFB < 0.3V), the 3701 F02 switching frequency of that channel is reduced to 1/5 of
Figure 2. Maximum Peak Current vs Duty Cycle
the normal operating frequency. The other controller channel is unaffected and maintains normal operation.
Power Good (PGOOD) Pin
This lower frequency allows the inductor current to safely A window comparator monitors both output voltages and discharge, thereby preventing current runaway. The switch- the open-drain PGOOD output is pulled low when the ing frequency will return to its normal value when the divided down output voltages are not within ±8% of the feedback voltage rises above 0.3V. During the first 64 reference voltage of 0.8V. PGOOD is pulled low when cycles (nonzero-current cycles) of soft-start, however, the channel 1 or both channels are shut down. When chan- controller operates at its full frequency. nel␣ 2 is shut down and channel 1 enabled, the PGOOD output indicates the state of channel 1 only.
Output Overvoltage Protection
As a further protection, the overvoltage comparator in the
2-Phase Operation
LTC3701 will turn the external MOSFET off when the The LTC3701 dual switching controller offers the consid- feedback voltage has risen 10% above the reference erable benefits of using 2-phase operation. Circuit ben- voltage of 0.8V. This comparator has a typical hysteresis efits include lower input filtering requirements, reduced of 20mV. electromagnetic interference (EMI) and increased effi- ciency associated with 2-phase operation. 3701fa 8
U OPERATIO (Refer to Functional Diagram) Dropout Operation Slope Compensation and Peak Inductor Current
When the input supply voltage decreases towards the The inductor’s peak current is determined by: output voltage, the rate of change of the inductor current during the ON cycle decreases. This reduction means that V 0 7V I ITH RUN PK = / – . the external P-channel MOSFET will remain on for more 10 • RSENSE than one oscillator cycle if the inductor current has not when the LTC3701 is operating below 20% duty cycle. ramped up to the threshold set by EAMP on the ITH/RUN However, once the duty cycle exceeds 20%, slope pin. Further reduction in input supply voltage will eventu- ally cause the P-channel MOSFET to be turned on 100%, compensation begins and effectively reduces the peak inductor current. The amount of reduction is given by the i.e., DC. The output voltage will then be determined by the curve in Figure 2. input voltage minus the voltage drop across the MOSFET, the sense resistor and the inductor. 110 100
Undervoltage Lockout
90 80 To prevent operation of the P-channel MOSFET below safe 70 input voltage levels, an undervoltage lockout is incorpo- (%) 60 MAX rated into the LTC3701. When the input supply voltage 50 drops below 2V, the P-channel MOSFET and all circuitry SF = I/I 40 are turned off except the undervoltage block, which draws 30 20 only several microamperes. 10 0
Short-Circuit Protection
0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) When an output is shorted to ground (VFB < 0.3V), the 3701 F02 switching frequency of that channel is reduced to 1/5 of
Figure 2. Maximum Peak Current vs Duty Cycle
the normal operating frequency. The other controller channel is unaffected and maintains normal operation.
Power Good (PGOOD) Pin
This lower frequency allows the inductor current to safely A window comparator monitors both output voltages and discharge, thereby preventing current runaway. The switch- the open-drain PGOOD output is pulled low when the ing frequency will return to its normal value when the divided down output voltages are not within ±8% of the feedback voltage rises above 0.3V. During the first 64 reference voltage of 0.8V. PGOOD is pulled low when cycles (nonzero-current cycles) of soft-start, however, the channel 1 or both channels are shut down. When chan- controller operates at its full frequency. nel␣ 2 is shut down and channel 1 enabled, the PGOOD output indicates the state of channel 1 only.
Output Overvoltage Protection
As a further protection, the overvoltage comparator in the
2-Phase Operation
LTC3701 will turn the external MOSFET off when the The LTC3701 dual switching controller offers the consid- feedback voltage has risen 10% above the reference erable benefits of using 2-phase operation. Circuit ben- voltage of 0.8V. This comparator has a typical hysteresis efits include lower input filtering requirements, reduced of 20mV. electromagnetic interference (EMI) and increased effi- ciency associated with 2-phase operation. 3701fa 8
