ISL8014A

FN7525 Rev 2.00

Page 12 of 16

December 16, 2014

Theory of Operation

The ISL8014A 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 ISL8014A employs the

current-mode pulse-width modulation (PWM) control scheme for

fast transient response and pulse-by-pulse current limiting. “Block

Diagram” is shown on page 3. 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 200mV/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 34 on

page 12 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.

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.

SKIP Mode

Pulling the SYNCH pin LO (<0.4V) forces the converter into PFM

mode. The ISL8014A enters a pulse-skipping mode at light load

to minimize the switching loss by reducing the switching

frequency. Figure 35 illustrates the skip-mode operation. A

zero-cross sensing circuit shown in the “Block Diagram” monitors

the N-MOSFET current for zero crossing. When 8 consecutive

cycles of the inductor current crossing zero are detected, the

regulator enters the skip mode. During the eight detecting cycles,

the current in the inductor is allowed to become negative. The

counter is reset to zero when the current in any cycle does not

cross zero.

Once the skip mode is entered, the pulse modulation starts being

controlled by the SKIP comparator shown in the “Block Diagram”.

Each pulse cycle is still synchronized by the PWM clock. The P-

MOSFET is turned on at the clock's rising edge and turned off when

the output is higher than 1.5% of the nominal regulation or when its

current reaches the peak skip current limit value, then the inductor

current is discharging to 0A and stays at zero. The internal clock is

disabled. The output voltage reduces gradually due to the load

current discharging the output capacitor. When the output voltage

drops to the nominal voltage, the P-MOSFET will be turned on again

at the rising edge of the internal clock as it repeats the previous

operations.

The regulator resumes normal PWM mode operation when the

output voltage drops 1.5% below the nominal voltage.

FIGURE 34. PWM OPERATION WAVEFORMS

DUTY

CYCLE

FIGURE 35. SKIP MODE OPERATION WAVEFORMS

CLOCK

NOMINAL +1.5%

NOMINAL

PFM CURRENT LIMIT

LOAD CURRENTT

0

PWM

PFM

8 CYCLES