5

Functional Pin Descriptions

VCC (Pin 11)

Provide a well decoupled 5V bias supply for the IC to this

pin. This pin also provides the gate bias charge for the lower

MOSFET controlled by the PWM section of the IC, as well as

the drive current for the linear regulators. The voltage at this

pin is monitored for Power-On Reset (POR) purposes.

GND (Pin 5)

Signal ground for the controller. All voltage levels are

measured with respect to this pin.

PGND (Pin 9)

This is the power ground connection. Tie the source of the

lower MOSFET of the synchronous PWM converter to this

pin.

BOOT (Pin 7)

Floating bootstrap supply pin for the upper gate drive. The

bootstrap capacitor provides the necessary charge to turn

and hold the upper MOSFET on. Connect a suitable

capacitor (0.47

µF recommended) from this pin to PHASE.

OCSET (Pin 12)

Connect a resistor from this pin to the drain of the upper

PWM MOSFET. This resistor, an internal 40

µA current

source (typical), and the upper MOSFET’s on-resistance set

the converter overcurrent trip point. An overcurrent trip

cycles the soft-start function.

The voltage at this pin is monitored for power-on reset

(POR) purposes and pulling this pin below 1.25V with an

open drain/collector device will shut down the switching

controller.

PHASE (Pin 6)

Connect this pin to the source of the PWM converter upper

MOSFET. This pin is used to monitor the voltage drop across

the upper MOSFET for overcurrent protection.

UGATE (Pin 8)

Connect UGATE pin to the PWM converter’s upper

MOSFET gate. This pin provides the gate drive for the upper

MOSFET.

LGATE (Pin 10)

This pin provides the gate drive for the synchronous rectifier

lower MOSFET. Connect LGATE to the gate of the lower

MOSFET.

COMP and FB (Pins 4, 3)

COMP and FB are the available external pins of the PWM

converter error amplifier. The FB pin is the inverting input of the

error amplifier. Similarly, the COMP pin is the error amplifier

output. These pins are used to compensate the voltage-mode

control feedback loop of the synchronous PWM converter.

DRIVE2, 3, 4 (Pins 1, 15, 13)

Connect these pins to the point of load or to the base

terminals of external bipolar NPN transistors. These pins are

each capable of providing 120mA of load current or drive

current for the pass transistors.

FB2, 3, 4 (Pins 2, 16, 14)

Connect the output of the corresponding linear regulators to

these pins through properly sized resistor dividers. The

voltage at these pins is regulated to 0.8V. These pins are

also monitored for undervoltage events.

Quickly pulling and holding any of these pins above 1.25V

(using diode-coupled logic devices) shuts off the respective

regulators. Releasing these pins from the pull-up voltage

initiates a soft-start sequence on the respective regulator.

Description

Operation

The ISL6521 monitors and precisely controls one

synchronous PWM converter and three configurable linear

regulators from a +5V bias input. The PWM controller is

designed to regulate the core voltage of an embedded

processor or simple down conversion for high current

applications. The PWM controller drives two MOSFETs (Q1

and Q2) in a synchronous-rectified buck converter

configuration and regulates the output voltage to a level

programmed by a resistor divider. The linear controllers are

designed to regulate three additional system voltages.

Typically, these include any I/O, memory, or clock voltages

that might be required. All three linear controllers support

up to 120mA of load current without external pass devices

or higher currents with external NPN bipolar transistors.

Initialization

The ISL6521 automatically initializes upon receipt of input

power. The Power-On Reset (POR) function continually

monitors the input bias supply voltage. The POR monitors

the bias voltage at the VCC pin. The POR function initiates

soft-start operation after the bias supply voltage exceeds its

POR threshold.

Soft-Start

The POR function initiates the soft-start sequence. The

PWM error amplifier reference input is clamped to a level

proportional to the soft-start voltage. As the soft-start voltage

slews up, the PWM comparator generates PHASE pulses of

increasing width that charge the output capacitor(s).

Similarly, all linear regulators’ reference inputs are clamped

to a voltage proportional to the soft-start voltage. The ramp-

up of the internal soft-start function provides a controlled

output voltage rise.

Figure 1 shows the soft-start sequence for a typical

application. At T0 the +5V bias voltage starts to ramp up

crossing the 4.5V POR threshold at time T1. On the PWM

section, the oscillator’s triangular waveform is compared to

ISL6521