ISL97656
6
FN6439.6
July 19, 2012
Applications Information
The ISL97656 is a high frequency, high efficiency boost regulator
operated at constant frequency PWM mode. The boost converter
stores energy from an input voltage source and delivers higher
output voltage. The input voltage range is 2.2V to 6.0V and the
output voltage range is 5V to 25V. The switching frequency can be
selected between 640kHz and 1.22MHz. The higher switching
frequency allows use of smaller inductors and faster transient
response. An external compensation pin gives the user greater
flexibility in setting output transient response and tighter load
regulation. The converter soft-start characteristic can be
controlled by the external C
SS
capacitor. The EN pin allows the
user to shut down the device.
Boost Converter Operations
Figure 13 shows a boost converter with all the key components.
In steady state and continuous conduction mode, the boost
converter operates in two cycles. During the first cycle, as shown
in Figure 14, the internal power FET turns on and the Schottky
diode is reverse biased and cuts off the current flow to the
output. The output current is supplied from the output capacitor.
The voltage across the inductor is V
IN
and the inductor current
ramps up with a rate of V
IN
/L, where L is the inductance. The
inductor is magnetized and energy is stored in the inductor. The
change in inductor current is shown in Equation 1:
During the second cycle, the power FET turns off and the
Schottky diode is forward biased, (see Figure 15). The energy
stored in the inductor is supplied to the output. This energy is
used to charge the output capacitor and supply output current. In
this cycle switching node (LX) is held to V
OUT
+ Schottky diode
drop. Voltage drop across the inductor is V
IN
- V
OUT
(ignoring
diode drop across Schottky diode). The change in inductor
current during the second cycle is shown in Equation 2:
In steady state operation, the change in the inductor current
must be equal 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 as
shown in Equation 4. The nominal VFB voltage is 1.24V.
ΔI
L1
ΔT1
V
IN
L
---------
×=
ΔV
O
I
OUT
C
OUT
----------------
ΔT
1
×=
(EQ. 1)
ΔI
L
ΔT2
V
IN
V
OUT
–
L
------------------------------- -
×=
ΔT2
1D–
f
SW
-------------
=
(EQ. 2)
D
f
SW
----------
V
IN
L
---------
1D–
f
SW
-------------
V
IN
V
OUT
–
L
------------------------------- -
×+× 0=
V
OUT
V
IN
----------------
1
1D–
-------------
=
(EQ. 3)
ISL97656
C
OUT
C
IN
LD
V
IN
V
OUT
FIGURE 13. BOOST CONVERTER
ISL97656
C
OUT
C
IN
L
V
IN
V
OUT
ΔT
1
ΔV
O
I
L
ΔI
L1
FIGURE 14. BOOST CONVERTER - CYCLE 1, POWER SWITCH CLOSED
ISL97656
C
OUT
C
IN
LD
V
IN
V
OUT
ΔT
2
ΔV
O
ΔI
L2
I
L
FIGURE 15. BOOST CONVERTER - CYCLE 2, POWER SWITCH OPEN
V
OUT
V
FB
1
R
1
R
2
-------
+
⎝⎠
⎜⎟
⎛⎞
×=
(EQ. 4)
