NCP1589A, NCP1589B

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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 k

W,

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 k

W 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

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.