ISL78233 datasheet(13/21 Pages) RENESAS

13 / 21 page

ISL78233, ISL78234

FN8359 Rev 8.00

Page 13 of 21

Apr 19, 2018

Theory of Operation

The ISL78233, ISL78234 are step-down switching regulators

optimized for automotive battery powered applications. The

regulator operates at a 2MHz default switching frequency for

high efficiency and allow smaller form factor, when FS is

connected to VIN. By connecting a resistor from FS to SGND, the

operational frequency adjustable range is 500kHz to 4MHz. 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 45µA. The supply current is

typically only 3.8µA when the regulator is shut down.

PWM Control Scheme

Pulling the SYNC pin HI (>0.8V) forces the converter into PWM

mode, regardless of output current. The ISL78233, ISL78234

employs the current-mode Pulse-Width Modulation (PWM)

control scheme for fast transient response and pulse-by-pulse

current limiting. Figure 3 on page 5 shows the functional 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 slope compensation is

440mV/Ts, which changes proportionally with frequency. 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

reaches the control reference of the current loop, the PWM

comparator COMP sends a signal to the PWM logic to turn off the

PFET and turn on the N-channel MOSFET. The NFET stays on until

the end of the PWM cycle. Figure 33 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 0.6V 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 55pF and 100kΩ RC network. The maximum EAMP

voltage output is precisely clamped to 2.5V.

Skip Mode

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

mode. The ISL78233, ISL78234 enters a Pulse-Skipping mode

at light load to minimize the switching loss by reducing the

switching frequency. Figure 34 on page 14 illustrates Skip mode

operation. A zero-cross sensing circuit shown in Figure 3 on

page 5 monitors the NFET current for zero crossing. When 16

consecutive cycles are detected, the regulator enters Skip mode.

During the sixteen 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 Skip mode is entered, the pulse modulation starts being

controlled by the Skip comparator shown in Figure 3 on page 5.

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

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

output is higher than 1.2% 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), and the output voltage reduces

gradually due to the load current discharging the output

capacitor. When the output voltage drops to the nominal voltage,

the PFET 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 1.2% below the nominal voltage.

FIGURE 33. PWM OPERATION WAVEFORMS

VEAMP

VCSA

DUTY

CYCLE

VOUT

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