ISL8023AIRTAJZ-T7A Intersil, ISL8023AIRTAJZ-T7A Datasheet - Page 14

• Manufacturer Part Number: ISL8023AIRTAJZ-T7A
• Description: ISL8023 Series 2.7 - 5.5 V 3 A 4 Mhz Buck Regulator - TQFN-16
• Manufacturer: Intersil

Theory of Operation
The ISL8023, ISL8024 is a step-down switching regulator
optimized for battery-powered handheld applications. The
regulator operates at 1MHz fixed default switching frequency,
when FS is connected to VIN, under heavy load conditions to
allow smaller external inductors and capacitors to be used for
minimal printed-circuit board (PCB) area. 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
50µA. The supply current is typically only 5µA 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 ISL8023, ISL8024
employs the current-mode pulse-width modulation (PWM) control
scheme for fast transient response and pulse-by-pulse current
limiting. Figure 4 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 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
P-FET and turn on the N-Channel MOSFET. The N-FET stays on
until the end of the PWM cycle. Figure 40 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 V
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 1.6V.
14
ISL8023, ISL8024
EAMP
voltage
SKIP Mode
Pulling the SYNC pin LO (<0.4V) forces the converter into PFM
mode. The ISL8023, ISL8024 enters a pulse-skipping mode at
light load to minimize the switching loss by reducing the
switching frequency. Figure 41 illustrates the skip-mode
operation. A zero-cross sensing circuit shown in Figure 4 on
page 5 monitors the N-FET 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 Figure 4 on page 5.
Each pulse cycle is still synchronized by the PWM clock. The
P-FET 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-FET 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.
CYCLE
V
V
DUTY
V
EAMP
CSA
OUT
I
L
FIGURE 40. PWM OPERATION WAVEFORMS
May 17, 2012
FN7812.2