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ISL97519A Datasheet(PDF) 7 Page - Intersil Corporation

Part No. ISL97519A
Description  600kHz/1.2MHz PWM Step-Up Regulator
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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ISL97519A Datasheet(HTML) 7 Page - Intersil Corporation

   
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7
FN6683.2
June 30, 2008
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 2.0A, the current limit of the
ISL97519A. 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 the input and
output voltages, the maximum output current, the inductor
value and the noise allowed to put back on the input line. For
most applications, a minimum 10µF is required. For
applications that run close to the maximum output current
limit, input capacitor in the range of 22µF to 47µF is
recommended.
The ISL97519A 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 ISL97519A incorporates a transconductance amplifier in
its feedback path to allow the user some adjustment on the
transient response and better regulation. The ISL97519A
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 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 0.5k to 7.5k and the compensation capacitor in
the range of 3nF to 10nF.
Soft-Start
During power-up, assuming EN is tied to VDD, as VDD rises
above VDD UVLO, the SS capacitor begins to charge up
with a constant 3µA current. During the time the part takes to
rise to 60mV the boost will not be enabled. Depending on
the value of the capacitor on the SS pin, this provides
sufficient (540µs for a 27nf capacitor or 2ms for a 100nf
capacitor) time for the passive in-rush current to settle down,
allowing the output capacitors to be charged to a diode drop
below VDD.
After the SS pin passes above the threshold beyond which
the part is enabled (60mV) the part begins to switch. The
linearly rising SS voltage, at a charge rate proportional to
3µA, has a direct effect on the current limit allowing the
current limit to linearly ramp-up to full current limit. SS
voltage of 200mV corresponds to a current limit around
350mA and 0.6V corresponds to full current limit.
The total soft-start time is calculated in Equation 7:
The full current is available after the soft-start period is
finished. The soft-start capacitor should be selected to be big
enough that it doesn't reach 0.6V before the output voltage
reaches the final value.
When the ISL97519A is disabled, the soft-start capacitor will
be discharged to ground.
Frequency Selection
The ISL97519A switching frequency can be user selected to
operate at either constant 620kHz or 1.25MHz. Connecting
FSEL pin to ground sets the PWM switching frequency to
620kHz. When connecting FSEL high or VDD, the switching
frequency is set to 1.25MHz.
Shutdown Control
When the EN pin is pulled down, the ISL97519A is shutdown
reducing the supply current to <1µA.
Maximum Output Current
The MOSFET current limit is nominally 2.0A and guaranteed
1.5A when VDD is greater than 2.8V. This restricts the
maximum output current, IOMAX, based on Equation 8:
where:
IL = MOSFET current limit
IL-AVG = average inductor current
ΔIL = inductor ripple current
VDIODE = Schottky diode forward voltage, typically, 0.6V
FS = switching frequency, 600kHz or 1.2MHz
D = MOSFET turn-on ratio:
Table 1 gives typical maximum IOUT values for 1.2MHz
switching frequency and 10µH inductor.
t
ss
Css
0.6V
×
3
μA
------------------------------
Css
2
10
×
×
5
==
(EQ. 7)
I
L
I
L-AVG
12
⁄ΔI
L
×
()
+
=
(EQ. 8)
ΔI
L
V
IN
V
O
V
DIODE
+
() V
IN
[]
×
LV
O
(
V
DIODE )
F
S
×
+
×
------------------------------------------------------------------------------
=
(EQ. 9)
I
L-AVG
I
OUT
1D
-------------
=
(EQ. 10)
D1
V
IN
V
OUT
V
DIODE
+
--------------------------------------------
=
(EQ. 11)
ISL97519A


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