AP3591 datasheet(13/18 Pages) DIODES

13 / 18 page

AP3591

Document number: DS36906 Rev. 1 - 2

13 of 18

www.diodes.com

March 2014

AP3591

A Product Line of

Diodes Incorporated

Application Information (Cont.)

7. Over Current Protection (OCP)

Figure 3 shows the over current protection (OCP) scheme of AP3591. In each switching cycle, the inductor current is sensed by monitoring the

low-side MOSFET in the OFF period. When the voltage between PGND pin and PHASE pin is larger than the over current trip level, the OCP is

triggered and the controller keeps the OFF state. Because the RDS(ON) of MOSFET increases with the temperature, IOC has 4500ppm/ºC

temperature coefficient to compensate this temperature dependency of RDS(ON).

Figure 3. Over Current Scheme

A resistor ROC-SET should be connected from CS pin to GND. An internal current source IOC (10µA typically), flowing through ROC-SET determines

the OCP trip point IOCSET, which can be calculated using the following equation:

The load current at over-current threshold (IO_OCSET), can be calculated using the following equation:

VOUT is the output voltage, ΔIL(PP) is the inductor current ripple peak to peak value and f is the switching frequency.

8. Negative Over Current Protection (NOCP)

The AP3591 supports cycle by cycle negative over current limiting in CCM mode. The over current limit value is the same absolute value as the

positive over current limit. If the inductor reverse current is larger than NOCP current at OFF time, the LMOSFET is turned off. The reverse current

will flows to VIN through the body diode of HMOSFET. After 400ns delay, LMOSFET is turned on again. If the NOCP is released, the HMOSFET

is turned on and the device resumes normal operation.

9. Under Voltage Protection (UVP)

The output voltage is also monitored for under voltage protection. When the output voltage is less than 70% of the setting voltage threshold, under

voltage protection is triggered after 28µs delay to prevent false transition. When UVP is triggered, UGATE and LGATE will get low, and the output

is discharged with the internal 20

Ω transistor. UVP is a latched protection; it can only be released by VDD or EN/DEM power-on reset. The UVP

blanking time is 2ms during soft-start.

10. Under Voltage Lockout

The AP3591 provides an under voltage lockout circuit to prevent from undefined status at startup. The UVLO circuit shuts down the device when

VDD drops below 3.6V. The UVLO circuit has 300mV hysteresis, which means the device will start up again when VDD rises to 3.9V.

11. Over Voltage Protection (OVP)

The feedback voltage is continuously monitored for over voltage protection. When OVP is triggered, LGATE will go high and UGATE will go low to

discharge the output capacitor.

COMPARATOR

CURRENT SENSE

Ioc

OPS

ROC-SET

REVERSE BUFFER

)

(

)

(

SET







OUT

















)

(

DS(ON)

)

(

DS(ON)

O_OCSET

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