ISL8013A

FN7526 Rev 2.01

Page 12 of 17

Dec 3, 2021

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 35

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 block only affects the operation during the

start-up and will be discussed separately. The error amplifier is a

transconductance amplifier that converts the voltage error signal

to a current output. The voltage loop is internally compensated

with the 27pF and 390kΩ RC network. The maximum EAMP

voltage output is precisely clamped to 1.6V.

FIGURE 32. OUTPUT SHORT-CIRCUIT

FIGURE 33. OUTPUT SHORT-CIRCUIT RECOVERY

FIGURE 34. OUTPUT CURRENT LIMIT vs TEMPERATURE

Typical Operating Performance Unless otherwise noted, operating conditions are: TA = +25°C, VIN = 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

0

255075

100

TEMPERATURE (°C)

OCP_3.3VIN

OCP_5VIN