ISL97656
7
FN6439.6
July 19, 2012
Inductor Selection
The inductor selection determines the output ripple voltage,
transient response, output current capability and efficiency. Its
selection depends on the input voltage, output voltage, switching
frequency and maximum output current. For most applications,
the inductance should be in the range of 2µH to 33µH. The
inductor maximum DC current specification must be greater than
the peak inductor current required by the regulator. The peak
inductor current can be calculated using Equation 5:
Output Capacitor
Low ESR capacitors should be used to minimize the output
voltage ripple. Multilayer ceramic capacitors (X5R and X7R) are
preferred for the output capacitors because of their lower ESR
and small packages. Tantalum capacitors with higher ESR can
also be used. The output ripple can be calculated using
Equation 6:
For noise sensitive applications, a 0.1µF placed in parallel with
the larger output capacitor is recommended to reduce the
switching noise coupled from the LX switching node.
Schottky Diode
In selecting the Schottky diode, the reverse break-down voltage,
forward current and forward voltage drop must be considered for
optimum converter performance. The diode must be rated to
handle 4.0A, the current limit of the ISL97656. The breakdown
voltage must exceed the maximum output voltage. Low forward
voltage drop, low leakage current, and fast reverse recovery will
help the converter to achieve the maximum efficiency.
Input Capacitor
The value of the input capacitor depends on the input and the
output voltages, maximum output current, inductor value and
maximum permissible noise fed back in the input line. For most
applications, a minimum 10µF is required. For applications that
run close to the maximum output current limit, an input
capacitor in the range of 22µF to 47µF is recommended.
The ISL97656 is powered from the VIN. A High frequency 0.1µF
bypass capacitor is recommended to be close to the VIN pin to
reduce supply line noise and ensure stable operation.
Loop Compensation
The ISL97656 incorporates a transconductance amplifier in its
feedback path to allow the user some adjustment on the
transient response and better regulation. The ISL97656 uses
current mode control architecture, which has a fast current sense
loop and a slow voltage feedback loop. The fast current feedback
loop does not require any compensation. The slow voltage loop
must be compensated for stable operation. The compensation
network is a series RC network from the COMP pin to ground. The
resistor sets the high frequency integrator gain for fast transient
response and the capacitor sets the integrator zero to ensure
loop stability. For most applications, the compensation resistor in
the range of 0k to 2.0k and the compensation capacitor in the
range of 3nF to 10nF.
Soft-Start
The regulator goes through the soft-start sequence after EN is
pulled high. The soft-start is provided by an internal 4.5µA
current source. This internal current source is used to charge the
external C
SS
capacitor. The peak MOSFET current is limited by
the voltage on the capacitor. As the voltage at the C
SS
capacitor
increases, this results in ramping up of the current limit from 0A
to full scale. This in turn controls the rising rate of the output
voltage.
Frequency Selection
The ISL97656 switching frequency can be user selectable. The
ISL97656 operates at either constant 640KHz or 1.22MHz
switching frequency. Connecting the FREQ pin to ground sets the
PWM switching frequency to 640kHz. When connecting FREQ
high or V
IN
, the switching frequency is set to 1.22MHz.
Shutdown Control
When the EN pin is pulled low, the ISL97656 is in shutdown
mode, reducing the supply current to <1µA.
Maximum Output Current
The MOSFET current limit is nominally 4.0A and guaranteed
3.8A. This restricts the maximum output current, I
OMAX
, based
on Equation 7:
where:
I
L
= MOSFET current limit
I
L(AVG)
= average inductor current
ΔI
L
= inductor ripple current
V
DIODE
= Schottky diode forward voltage, typically, 0.6V
f
SW
= switching frequency, 640kHz or 1.22MHz
D = MOSFET turn-on ratio:
I
L PEAK()
I
OUT
V
OUT
×
V
IN
------------------------------------
12⁄
V
IN
V
OUT
V
IN
–()×
LV
OUT
FREQ××
---------------------------------------------------- -
×+=
(EQ. 5)
ΔV
O
I
OUT
D×
f
SW
C
O
×
-------------------------
I
OUT
ESR×+=
(EQ. 6)
I
L
I
LAVG()
12⁄ΔI
L
×()+=
(EQ. 7)
ΔI
L
V
IN
V
O
V
DIODE
+()V
IN
–[]×
LV
O
( V
DIODE
) f
SW
×+×
------------------------------------------------------------------------------
=
(EQ. 8)
I
L-AVG
I
OUT
1D–
-------------
=
(EQ. 9)
D1
V
IN
V
OUT
V
DIODE
+
--------------------------------------------
–=
(EQ. 10)
