ISL8013A datasheet(12/17 Pages) INTERSIL

12 / 17 page

FN7526.0

November 25, 2009

Theory of Operation

The ISL8013A is a step-down switching regulator

optimized for battery-powered handheld applications.

The regulator operates at 1MHz fixed switching

frequency under heavy load conditions to allow smaller

external inductors and capacitors to be used for minimal

printed-circuit board (PCB) area. At light load, the

regulator reduces the switching frequency, unless forced

to the fixed frequency, to minimize the switching loss and

to maximize the battery life. The quiescent current when

the output is not loaded is typically only 35µA. The

supply current is typically only 0.1µA when the regulator

is shut down.

PWM Control Scheme

Pulling the SYNCH pin HI (>2.5V) forces the converter

into PWM mode, regardless of output current. The

ISL8013A employs the current-mode pulse-width

modulation (PWM) control scheme for fast transient

response and pulse-by-pulse current limiting. Figure 2

shows the block diagram. The current loop consists of the

oscillator, the PWM comparator, current sensing circuit

and the slope compensation for the current loop stability.

The gain for the current sensing circuit is typically

250mV/A. The control reference for the current loops

comes from the error amplifier's (EAMP) output.

The PWM operation is initialized by the clock from the

oscillator. The P-Channel MOSFET is turned on at the

beginning of a PWM cycle and the current in the

MOSFET starts to ramp up. When the sum of the current

amplifier CSA and the slope compensation (237mV/µs)

reaches the control reference of the current loop, the

PWM comparator COMP sends a signal to the PWM logic

to turn off the P-MOSFET and turn on the N-Channel

MOSFET. The N-MOSFET stays on until the end of the

PWM cycle. Figure 36 shows the typical operating

waveforms during the PWM operation. The dotted lines

illustrate the sum of the slope compensation ramp and

the current-sense amplifier’s CSA output.

The output voltage is regulated by controlling the VEAMP

voltage to the current loop. The bandgap circuit outputs

a 0.8V reference voltage to the voltage loop. The

feedback signal comes from the VFB pin. The soft-start

FIGURE 33. OUTPUT SHORT CIRCUIT

FIGURE 34. OUTPUT SHORT CIRCUIT RECOVERY

FIGURE 35. OUTPUT CURRENT LIMIT vs TEMPERATURE

Typical Operating Performance Unless otherwise noted, operating conditions are: T

A = +25°C,

VVIN = 2.5V to 5.5V, EN = VIN, SYNCH = 0V, L = 1.5µH,

C1 = 2x22µF, C2 = 2x22µF, IOUT = 0A to 3A. (Continued)

PHASE 2V/DIV

VOUT 0.5V/DIV

IL 2A/DIV

PG 5V/DIV

LX 2V/DIV

VOUT 1V/DIV

IL 2A/DIV

PG 5V/DIV

4.000

4.125

4.250

4.375

4.500

4.625

4.750

4.875

5.000

-50

-25

255075

100

TEMPERATURE (°C)

OCP_3.3VIN

OCP_5VIN

ISL8013A

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