ADP1851

Data Sheet

Rev. B | Page 14 of 24

SHORT-CIRCUIT AND CURRENT-LIMIT

PROTECTION

When the output is shorted or the output current exceeds the

current limit set by the current-limit setting resistor (between

ILIM and SW) for eight consecutive cycles, the ADP1851 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 3 kΩ resistor

during an overcurrent or short-circuit event. Figure 22 shows

that the ADP1851 on a high current application circuit maintains

current-limit hiccup mode when the output is shorted.

M 4ms 2.5MS/s

400ns/pt

A CH4

18.2V

B

W

2

3

CH3 10V

CH2 1V

CH4 10A

B

W

B

W

SW

VOUT

INDUCTOR

CURRENT

4

Ω

Figure 22. Current-Limit Hiccup Mode, 30 A Current Limit

ENABLE/DISABLE CONTROL

The EN pin is used to enable or disable the ADP1851 controller;

the typical precision enable threshold is 0.63 V. When the voltage at

EN rises above the threshold voltage, the controller is enabled and

starts normal operation after initialization of the internal oscillator,

references, settings, and the soft start period. When the voltage

at EN falls to typically 30 mV (hysteresis) below the threshold

voltage, the driver and the internal controller circuits in the

ADP1851 are turned off. The initial settings are still valid;

therefore, reenabling the controller does not change the settings

until the power at the VIN pin is cycled. In addition, the EN

signal does not shut down the LDO regulator at VCCO, which

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

ADP1851 can be programmed by connecting an appropriate

resistor divider from the master power supply to the EN pin, as

shown in Figure 23. For example, 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.

ADP1851

FB

EN

MASTER

SUPPLY

VOLTAGE

R2

R1

Figure 23. Optional Power-Up Sequencing Circuit

THERMAL OVERLOAD PROTECTION

The ADP1851 has an internal temperature sensor that senses the

junction temperature of the chip. When the junction tempera-

ture of the ADP1851 reaches approximately 155°C, the ADP1851

goes into thermal shutdown, the converter is turned off, and the

SS/TRK pin discharges toward zero through an internal 3 kΩ

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

junction temperature falls below 135°C, the ADP1851 resumes

normal operation after the soft start sequence.