NCP1589A, NCP1589B
www.onsemi.com
6
APPLICATIONS INFORMATION
Over Current Protection (OCP)
The NCP1589A/B monitors the voltage drop across the
low side mosfet and uses this information to determine if
there is excessive output current. The voltage across the low
side mosfet is measured from the LX pin, and is referenced
to ground. The over current measurement is timed to occur
at the end of the low side mosfet conduction period, just
before the bottom mosfet is turned off.
If the voltage drop across the bottom mosfet exceeds the
over current protection threshold, then an internal counter is
incremented. If the voltage drop does not exceed the over
current protection threshold, then the internal counter is
reset. The NCP1589A/B will latch the over current
protection fault condition only if the over current protection
threshold is exceeded for four consecutive cycles.
When the NCP1589A/B latches an over current
protection fault, both the high side and low side mosfets are
turned off. To reset the over current protection fault, the
power to the VCC pin must be cycled.
The over current threshold voltage can be externally, by
varying the value of the ROCSET resistor. The ROCSET
resistor is a resistor connected between the LG pin (low side
mosfet gate) and ground.
During startup, after the VCC and BOOT pins reach the
under voltage lock out threshold, the NCP1589A/B will
source 10 mA of current out of the LG pin. This current will
flow through the ROCSET resistor and produce a voltage
that is sampled and then used as the over current protection
threshold voltage. For example, if ROCSET is set to 10 kW,
the 10 mA of current will yield a 100 mV threshold, and if the
voltage drop across the low side mosfet exceeds 100 mV at
the end of its conduction period, then an over current event
will be detected.
If the ROCSET resistor is not present, then the over
current protection threshold will max out at 640 mV. The
valid range for ROCSET is 5 kW to 55 kW which yields a
threshold voltage range of 50 mV to 550 mV.
Internal Soft-Start
To prevent excess inrush current during startup, the
NCP1589A/B uses a calibrated current source with an
internal soft start capacitor to ramp the reference voltage
from 0 to 800 mV over a period of 4 ms. The softstart ramp
generator will reset if the input power supply voltages reach
the under voltage lockout threshold, or if the NCP1589A/B
is disabled by having the COMP pin pulled low.
Startup into a Precharged Load
During a startup and soft start sequence the NCP1589A
will detect a residual charge on the output capacitors and not
forcefully discharge the capacitors before beginning the
softstart sequence, instead, the softstart ramping of the
output will begin at the voltage level of the residual charge.
For example, if the NCP1589A/B is configured to provide
a regulated output voltage of 2.5 V, the normal softstart
sequence will ramp the output voltage from 0 to 2.5 V in
4.2 ms; however if the output capcitors already have a 1.2 V
charge on them, the NCP1589A/B will not discharge the
capacitors, instead the softstart sequence will begin at 1.2 V
and then ramp the output to 2.5 V.
Power Good
The PGOOD pin is an open drain active high output pin
that signals the condition of the VOS (Voltage Output Sense)
pin. PGOOD is pulled low during soft start cycle, and if there
is a latched over current, over voltage, or under voltage fault.
If the voltage on the VOS pin is within ±10% of Vref
(800 mV) then the PGOOD pin will not be pulled low. The
PGOOD pin does not have an internal pull-up resistor.
Overvoltage Protection
If the voltage on the VOS pin exceeds the over voltage
threshold the NCP1589A/B will latch an over voltage fault.
During an over voltage fault the UG pin will be pulled low,
and the LG pin will be high while the until the voltage on the
VOS pin goes below V
ref
/2 (400 mV). The NCP1589A will
continue drive the LG pin, LG will go high if VOS exceeds
1 V and then go low when VOS goes below 400 mV. The
power to the NCP1589 must be cycled to reset the over
voltage protection fault.
Under Voltage Protection
If the voltage on the VOS pin falls below the under voltage
threshold after the soft start cycle completes, then the
NCP1589A/B will latch an under voltage fault. During an
under voltage fault, both the UG and LG pins will be pulled
low. The power to the NCP1589 must be cycled to reset the
under voltage protection fault.
