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ISL97519A Datasheet(PDF) 6 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) 6 Page - Intersil Corporation

   
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6
FN6683.2
June 30, 2008
During the second cycle, the power FET turns off and the
Schottky diode is forward biased, (see Figure 13). The
energy stored in the inductor is pumped to the output
supplying output current and charging the output capacitor.
The Schottky diode side of the inductor is clamped to a
Schottky diode above the output voltage. So the voltage
drop across the inductor is VIN - VOUT. The change in
inductor current during the second cycle is shown in
Equation 2:
For stable operation, the same amount of energy stored in
the inductor must be taken out. The change in inductor
current during the two cycles must be the same as shown in
Equation 3.
Output Voltage
An external feedback resistor divider is required to divide the
output voltage down to the nominal 1.24V reference voltage.
The current drawn by the resistor network should be limited
to maintain the overall converter efficiency. The maximum
value of the resistor network is limited by the feedback input
bias current and the potential for noise being coupled into
the feedback pin. A resistor network less than 100k is
recommended. The boost converter output voltage is
determined by the relationship in Equation 4:
The nominal VFB voltage is 1.24V.
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 in
Equation 5:
Output Capacitor
Low ESR capacitors should be used to minimized the output
voltage ripple. Multi-layer 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
as shown in Equation 6:
For noise sensitive application, a 0.1µF placed in parallel
with the larger output capacitor is recommended to reduce
the switching noise coupled from the LX switching node.
ΔI
L
ΔT2
V
IN
V
OUT
L
--------------------------------
×
=
ΔT2
1D
F
SW
-------------
=
(EQ. 2)
ΔI1 ΔI2
+
0
=
D
F
SW
------------
V
IN
L
---------
1D
F
SW
-------------
V
IN
V
OUT
L
--------------------------------
×
+
×
0
=
V
OUT
V
IN
----------------
1
1D
-------------
=
(EQ. 3)
ISL97519A
COUT
CIN
LD
VIN
VOUT
COUT
CIN
LD
VIN
VOUT
FIGURE 11. BOOST CONVERTER
COUT
CIN
L
VIN
VOUT
ΔT1
ΔV
O
IL
ΔIL1
ISL97519A
FIGURE 12. BOOST CONVERTER - CYCLE 1, POWER
SWITCH CLOSE
ISL97519A
COUT
CIN
LD
VIN
VOUT
ΔT2
ΔVO
ΔIL2
IL
FIGURE 13. BOOST CONVERTER - CYCLE 2, POWER
SWITCH OPEN
V
OUT
V
FB
1
R
1
R
2
-------
+
⎝⎠
⎜⎟
⎛⎞
×
=
(EQ. 4)
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)
ISL97519A


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