ADP2387ACPZN-R7 datasheet(13/25 Pages) AD

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Data Sheet

ADP2387

Rev. C | Page 13 of 25

POWER GOOD

The power-good pin (PGOOD) is an active high, open-drain

output that requires an external resistor to pull it up to a

voltage. A logic high on the PGOOD pin indicates that the

voltage on the FB pin (and, therefore, the output voltage) is

within regulation.

The power-good circuitry monitors the output voltage on the

FB pin and compares it to the rising and falling thresholds that

are specified in Table 1. If the rising output voltage exceeds the

target value, the PGOOD pin is held low. The PGOOD pin

continues to be held low until the falling output voltage returns

to the target value.

If the output voltage falls below the target output voltage, the

PGOOD pin is held low. The PGOOD pin continues to be held

low until the rising output voltage returns to the target value.

The power-good rising and falling thresholds are shown in

Figure 30. There is a 1024 clock cycle waiting period before

the PGOOD pin is pulled from low to high, and there is a

16 clock cycle waiting period before the PGOOD pin is pulled

from high to low.

100%

116.7%

105%

90%

95%

PGOOD

VOUT RISING

VOUT FALLING

1024 CYCLE

DEGLITCH

16 CYCLE

DEGLITCH

1024 CYCLE

DEGLITCH

16 CYCLE

DEGLITCH

Figure 30. PGOOD Rising and Falling Thresholds

PEAK CURRENT-LIMIT AND SHORT-CIRCUIT

PROTECTION

The ADP2387 has a peak current-limit protection circuit to

prevent current runaway. A resistor between ILIM and GND

programs the peak current-limit threshold according to the

following equation:

( )

kΩ

405

(A)

ILIM

OCP

where:

IOCP is the peak current-limit threshold.

RILIM is the resistor between ILIM and GND.

Figure 31 shows the typical relationship between the peak

current-limit threshold and RILIM.

During the initial soft start, the ADP2387 uses frequency

foldback to prevent output current runaway. The switching

frequency reduces according to the voltage on the FB pin,

which allows more time for the inductor to discharge. Table 5

shows the correlation between the switching frequency and

FB pin voltage.

RILIM (kΩ)

100

120

140

160

180

200

Figure 31. Peak Current-Limit Threshold vs. RILIM

Table 5. FB Pin Voltage and Switching Frequency

FB Pin Voltage

Switching Frequency

VFB ≥ 0.4 V

fSW

0.4 V > VFB ≥ 0.2 V

fSW/2

VFB < 0.2 V

fSW/4

For protection against heavy loads, the ADP2387 uses a hiccup

mode for overcurrent protection. When the inductor peak current

reaches the current-limit value, the high-side MOSFET turns

off and the low-side MOSFET turns on until the next cycle.

The overcurrent counter increments during this process. If the

overcurrent counter reaches 10 or if the FB pin voltage falls to

0.4 V after the soft start, the regulator enters hiccup mode. The

high-side and low-side MOSFETs both turn off. The regulator

remains in hiccup mode for 4096 clock cycles and then attempts

to restart. If the current-limit fault clears, the regulator resumes

normal operation. Otherwise, it reenters hiccup mode.

The ADP2387 also provides a sink current-limit to prevent the

low-side MOSFET from sinking excessive current from the load.

When the voltage across the low-side MOSFET exceeds the sink

current-limit threshold, which is typically 2.5 A, the low-side

MOSFET turns off immediately for the rest of the cycle. Both high-

side and low-side MOSFETs turn off until the next clock cycle.

In some cases, the input voltage (VPVIN) ramp rate is too slow or

the output capacitor is too large for the output to reach regulation

during the soft start process, which causes the regulator to enter

hiccup mode. To avoid such occurrences, use a resistor divider

at the EN pin to program the input voltage UVLO, or use a

longer soft start time.

OVERVOLTAGE PROTECTION (OVP)

The ADP2387 includes an overvoltage protection feature to

protect the regulator against an output short to a higher voltage

supply or when a strong load disconnect transient occurs. If the

feedback voltage increases to 0.7 V, the internal high-side and

low-side MOSFETs turn off until the voltage at the FB pin

decreases to 0.63 V. At that time, the ADP2387 resumes normal

operation.

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