Datasheet

ADP1877 Data Sheet
Rev. D | Page 16 of 32
SHORT-CIRCUIT AND CURRENT-LIMIT
PROTECTION
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.
When the output is shorted or the output current exceeds the
current limit set by the R
ILIM
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.
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.
CH3 500mV
CH1 10V
CH4 10A Ω
M2ms A CH1 11.2V
SW1
SS1
INDUCTOR CURRENT
08299-045
1
3
4
V
OUT1
R1
R2
ADP1877
EN1
OR
EN2
MASTER
SUPPLY
VOLTAGE
FB1
OR
FB2
R
TOP
R
BOT
08299-007
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.
Figure 29. Current Limit Hiccup Mode, 20 A Circuit