9
FN9256.0
March 7, 2006
Theory of Operation
The ISL6273 is a step-down switching regulator optimized
for battery-powered handheld applications. The regulator
operates at 1.5MHz fixed switching frequency under heavy
load condition to allow small external inductor 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 25μA. The supply current is typically only 0.1μA when
the regulator is shut down. The ISL6273 has four fixed
output voltage versions and one adjustable version.
PWM Control Scheme
The ISL6273 employs the current-mode pulse-width
modulation (PWM) control scheme for fast transient
response and pulse-by-pulse current limiting. Figure 17
shows the block diagram. The current loop consists of the
oscillator, the PWM comparator COMP, current sensing
circuit, and the slope compensation for the current loop
stability. The current sensing circuit consists of the
resistance of the P-channel MOSFET when it is turned on
and the current sense amplifier CSA. The gain for the
current sensing circuit is typically 0.4V/A. The control
reference for the current loops comes from the error
amplifier EAMP of the voltage loop.
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 compensation slope (0.675V/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 to turn on the N-channel MOSFET. The
N-MOSFET stays on until the end of the PWM cycle.
Figure 18 shows the typical operating waveforms during the
PWM operation. The dotted lines illustrate the sum of the
compensation ramp and the current-sense amplifier CSA
output.
The output voltage is regulated by controlling the reference
voltage to the current loop. The bandgap circuit outputs a
0.8V reference voltage to the voltage control loop. The
feedback signal comes from the FB pin. The soft-start block
only affects the operation during the start-up and will be
discussed separately shortly. 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 30pF and 300kΩ RC network. The
maximum EAMP voltage output is precisely clamped to the
bandgap voltage (1.172V).
SKIP Mode
The ISL6273 enters a pulse-skipping mode at light load to
minimize the switching loss by reducing the effective
switching frequency. Figure 19 illustrates the skip-mode
operation. A zero-cross sensing circuit shown in Figure 17
monitors the N-MOSFET current for zero crossing. When 8
consecutive cycles of the N-MOSFET crossing zero are
detected, the regulator enters the skip mode. During the 8
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.
FIGURE 18. PWM OPERATION WAVEFORMS
V
EAMP
V
CSA1
Duty
Cycle
I
L
V
OUT
FIGURE 19. SKIP MODE OPERATION WAVEFORMS
Clock
I
L
V
OUT
Nominal + 1.5%
Nominal
Current Limit
Load Current
0
8 Cycles
ISL6273
