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ISL8010 Datasheet PDF : 11 Pages
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ISL8010
Applications Information
Product Description
The ISL8010 is a synchronous, integrated FET 600mA
step-down regulator, which operates from an input of 2.5V to
5.5V. The output voltage is user-adjustable with a pair of
external resistors.
When the load is very light, the regulator automatically
operates in the PFM mode, thus achieving high efficiency at
light load (>70% for 1mA load). When the load increases,
the regulator automatically switches over to a voltage-mode
PWM operating at nominal 1.4MHz switching frequency. The
efficiency is up to 95%.
It can also operate in a fixed PWM mode or be synchronized
to an external clock up to 12MHz for improved EMI
performance.
PFM Operation
The heart of the ISL8010 regulator is the automatic
PFM/PWM controller.
If the SYNC pin is connected to ground, the regulator
operates automatically in either the PFM or PWM mode,
depending on load. When the SYNC pin is connected to VIN,
the regulator operates in the fixed PWM mode. When the pin
is connected to an external clock ranging from 1.6MHz to
12MHz, the regulator is in the fixed PWM mode and
synchronized to the external clock frequency.
In the automatic PFM/PWM operation, when the load is light,
the regulator operates in the PFM mode to achieve high
efficiency. The top P-Channel MOSFET is turned on first.
The inductor current increases linearly to a preset value
before it is turned off. Then the bottom N-Channel MOSFET
turns on, and the inductor current linearly decreases to zero
current. The N-Channel MOSFET is then turned off, and an
anti-ringing MOSFET is turned on to clamp the LX pin to VO.
Both MOSFETs remain off until VFB drops below the internal
reference voltage of 0.8V. The inductor current looks like
triangular pulses. The frequency of the pulses is mainly a
function of output current. The higher the load, the higher the
frequency of the pulses until the inductor current becomes
continuous. At this point, the controller automatically
changes to PWM operation.
When the controller transitions to PWM mode, there can be
a perturbation to the output voltage. This perturbation is due
to the inherent behavior of switching converters when
transitioning between two control loops. To reduce this
effect, it is recommended to use the phase-lead capacitor
(C4) shown in the “Typical Application Diagram” on page 1.
This capacitor allows the PWM loop to respond more quickly
to this type of perturbation. To properly size C4, refer to
“Component Selection” on page 10.
FN6191 Rev 6.00
October 18, 2010
PWM Operation
The regulator operates the same way in the forced PWM or
synchronized PWM mode. In this mode, the inductor current
is always continuous and does not stay at zero.
In this mode, the P-Channel MOSFET and N-Channel
MOSFET always operate complementary. When the
P-Channel MOSFET is on and the N-Channel MOSFET off,
the inductor current increases linearly. The input energy is
transferred to the output and also stored in the inductor.
When the P-Channel MOSFET is off and the N-Channel
MOSFET on, the inductor current decreases linearly, and
energy is transferred from the inductor to the output. Hence,
the average current through the inductor is the output
current. Since the inductor and the output capacitor act as a
low pass filter, the duty cycle ratio is approximately equal to
VO divided by VIN.
The output LC filter has a second order effect. To maintain
the stability of the converter, the overall controller must be
compensated. This is done with the fixed internally
compensated error amplifier and the PWM compensator.
Because the compensations are fixed, the values of input
and output capacitors are 10µF to 22µF ceramic and
inductor is 1.5µH to 2.2µH.
Forced PWM Mode/SYNC Input
Pulling the SYNC pin HI (>2.5V) forces the converter into
PWM mode in the next switching cycle regardless of output
current. The duration of the transition varies depending on the
output current. Figures 22 and 23 (under two different loading
conditions) show the device goes from PFM to PWM mode.
Note: In forced PWM mode, the IC will continue to start-up in
PFM mode to support pre-biased load applications.
Start-Up and Shutdown
When the EN pin is tied to VIN and VIN reaches
approximately 2.4V, the regulator begins to switch. The
inductor current limit is gradually increased to ensure proper
soft-start operation.
When the EN pin is connected to a logic low, the ISL8010 is
in the shutdown mode. All the control circuitry and both
MOSFETs are off, and VOUT falls to zero. In this mode, the
total input current is less than 1µA.
When the EN reaches logic HI, the regulator repeats the
start-up procedure, including the soft-start function.
Current Limit and Short-Circuit Protection
The current limit is set at about 1.2A for the PMOS. When a
short-circuit occurs in the load, the preset current limit restricts
the amount of current available to the output, which causes
the output voltage to drop as load demand increases. When
the output voltage drops 30mV below the reference voltage,
the converter will shutdown for a period of time (approximated
by Equation 1) and then restart. If the overcurrent condition
still exists, it will repeat the shutdown-wait-restart event. This
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