Datasheet

ADP1851 Data Sheet
Rev. 0 | 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.
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 is always active when V
IN
is above the
UVLO threshold.
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.
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
temperature 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.
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
10595-022
ADP1851
FB
EN
R
BOT
R
TOP
MASTER
SUPPLY
VOLTAGE
V
OUT
R2
R1
10595-023