ISL8510EVAL1Z【PDF 16/21 页】EVALUATION BOARD FOR ISL8510

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ISL8510

LDO Current Limit

The ISL8510 monitors and controls the pass transistor ’s

gate voltage to limit output current. The current limit for LDO

is 700mA typical. The output can be shorted to ground

without damaging the part due to the current limit and

thermal protection features.

Undervoltage Protection

If the voltage detected on the buck regulator FB pin falls 15%

below the internal reference voltage, the undervoltage fault

condition flag is set. The fault protection circuitry checks the

overcurrent flag. If the overcurrent flag is set, the fault

monitor latches off the internal power MOSFET. The

regulator will not restart until either a POR restart or the

EN_PWM pin is cycled.

If the overcurrent flag is not set, an internal undervoltage

counter is set to 1. The fault controller continues to monitor

the FB pin for 4 clock cycles. If the fault condition persists,

the regulator is shutdown. The controller enters a recovery

mode similar to the overcurrent hiccup mode. No action is

taken for 4 soft-start cycles and the internal undervoltage

counter and fault condition flag are reset. A normal soft-start

cycle is attempted and normal operation continues if the fault

condition has cleared. If the undervoltage counter overflows

during soft-start, the converter is shut down and this hiccup

mode operation repeats.

Undervoltage protection only applies to the buck regulator

output; the LDO output does not have undervoltage

protection.

Thermal Overload Protection

Thermal overload protection limits total power dissipation in

the ISL8510. There are three sensors on the chip to monitor

the junction temperature of the internal LDO, PWM switching

power N-Channel MOSFET, and LDO pass transistors.

When the junction temperature (T J ) of any of the three

sensors exceeds +150°C, the thermal sensor sends a signal

to the fault monitor.

The fault monitor commands the buck regulator to shut down

and the LDO to turn off the pass transistor. The buck

regulator soft-starts and the LDO pass transistor turn on

again after the IC’s junction temperature cools by +20°C.

The buck regulator experiences hiccup mode operation and

the LDO a pulsed output during continuous thermal overload

conditions. For continuous operation, do not exceed the

+125°C junction temperature rating.

Low Dropout Regulator

The regulator consists of a 0.6V reference, error amplifier,

MOSFET driver, P-Channel pass transistor, and dual-mode

comparator. The voltage is set by means of an external

resistor divider on the FB_LDO pin. The 0.6V band gap

reference is connected to the error amplifier ’s inverting input.

The error amplifier compares this reference to the selected

feedback voltage and amplifies the difference. The MOSFET

driver reads the error signal and applies the appropriate

drive to the P-Channel pass transistor. If the feedback

voltage is lower than the reference voltage, the pass

transistor gate is pulled lower, allowing more current to pass

and increasing the output voltage. If the feedback voltage is

higher than the reference voltage, the pass transistor gate is

driven higher, allowing less current to pass to the output.

Internal P-Channel Pass Transistor

The LDO regulator in the ISL8510 feature a typical 0.33 Ω

r DS(ON) P-Channel MOSFET pass transistor. This provides

several advantages over similar designs using PNP bipolar

pass transistors. The P-Channel MOSFET requires no base

drive, which reduces quiescent current considerably. PNP

based regulators waste considerable current in dropout

when the pass transistor saturates. They also use high base

drive currents under large loads. The ISL8510 does not have

these drawbacks.

Integrator Circuitry

The ISL8510 uses external compensation capacitors for

minimizing load and line regulation errors and for lowering

output noise. When the output voltage shifts due to varying

load current or input voltage, the integrator capacitor voltage

is raised or lowered to compensate for the systematic offset

at the error amplifier. Compensation is limited to ±5% to

minimize transient overshoot when the device goes out of

dropout, current limit, or thermal shutdown. Place a 33nF

capacitor to GND from CC1 and CC2.

Application Guidelines

Operating Frequency

The ISL8510 operates at a fixed switching frequency of

500kHz.

LDO Regulator Capacitor Selection

Capacitors are required at the ISL8510 LDO Regulators’

input and output for stable operation over the entire load

range and the full temperature range. Use a >1μF capacitor

at the input of LDO Regulator, VIN_LDO pin. The input

capacitor lowers the source impedance of the input supply.

Larger capacitor values and lower ESR provide better PSRR

and line transient response. The input capacitor must be

located at a distance of not more then 0.5 inches from the

VIN_LDO pin of the IC and returned to a clean analog

ground. Any good quality ceramic capacitor can be used as

an input capacitor.

The output capacitors used in LDO regulator are used to

provide dynamic load current. The amount of capacitance

and type of capacitor should be chosen with this criteria in

mind. The output capacitor selected must meet the

requirements of minimum amount of capacitance and ESR

for LDO. The ISL8510 is specifically designed to work with

small ceramic output capacitors. The output capacitor ’s ESR

affects stability and output noise. Use an output capacitor

FN6516.2

December 15, 2008

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