ADP1877ACPZ-R7 datasheet(16/32 Pages) AD

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ADP1877

Rev. 0 | Page 16 of 32

SHORT CIRCUIT AND CURRENT LIMIT

PROTECTION

When the output is shorted or the output current exceeds the

current limit set by the RILIM resistor for eight consecutive cycles,

the ADP1877 shuts off both the high-side and low-side drivers

and restarts the soft start sequence every 10 ms, which is known

as hiccup mode. The SS node discharges to zero through an

internal 1 kΩ resistor during an overcurrent or short-circuit

event. Figure 29 shows that the ADP1877 (a 20 A application

circuit) is entering current limit hiccup mode when the output

is shorted.

CH3 500mV

CH1 10V

CH4 10A Ω

M2ms

A CH1

11.2V

SW1

SS1

INDUCTOR CURRENT

Figure 29. Current Limit Hiccup Mode, 20 A Circuit

SHUTDOWN CONTROL

The EN1 and EN2 pins are used to enable or disable Channel 1

and Channel 2, respectively, of the ADP1877. The precision enable

threshold for EN1/EN2 is typically 0.63 V. When the EN1/EN2

voltage rises above 0.63 V, the ADP1877 is enabled and starts

normal operation after the soft start period. When the voltage

at EN1/EN2 drops below 0.57 V, the switchers and the internal

circuits in the ADP1877 are turned off. Note that EN1/EN2

cannot shut down the LDO at VCCO, which is always active.

For the purpose of start-up power sequencing, the startup of the

ADP1877 can be programmed by connecting an appropriate

resistor divider from the master power supply to the EN1/EN2

pin, as shown in Figure 30. For instance, if the desired start-up

voltage from the master power supply is 10 V, R1 and R2 can be

set to 156 kΩ and 10 kΩ, respectively.

VOUT1

ADP1877

EN1

EN2

MASTER

SUPPLY

VOLTAGE

FB1

FB2

RTOP

RBOT

Figure 30. Optional Power-Up Sequencing Circuit

THERMAL OVERLOAD PROTECTION

The ADP1877 has an internal temperature sensor that senses

the junction temperature of the chip. When the junction

temperature of the ADP1877 reaches approximately 155°C, the

ADP1877 goes into thermal shutdown, the converter is turned

off, and SS discharges toward zero through an internal 1 kΩ

resistor. At the same time, VCCO discharges to zero. When the

junction temperature drops below 135°C, the ADP1877 resumes

normal operation after the soft start sequence.

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