isl97656 Intersil Corporation, isl97656 Datasheet - Page 5

• Manufacturer Part Number: isl97656
• Description: 640khz/1.2mhz Pwm Step-up Regulator
• Manufacturer: Intersil Corporation

Typical Performance Curves
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 deliver it to a higher output voltage. The input voltage
range is 2.3V to 5.5V and output voltage range is 5V to 25V.
The switching frequency is selectable between 640kHz and
1.2MHz allowing 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 also be controlled by external C
allows the user to completely shutdown the device.
Boost Converter Operations
Figure 8 shows a boost converter with all the key
components. In steady state operating and continuous
conduction mode where the inductor current is continuous,
the boost converter operates in two cycles. During the first
cycle, as shown in Figure 9, 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
the inductance. The inductance is magnetized and energy is
stored in the inductor. The change in inductor current is
shown in Equation 1:
ΔV
ΔI
D
ΔT1
IN
L1
=
O
and the inductor current ramps up in a rate of V
Duty Cycle
=
=
=
ΔT1
--------- -
f
--------------- -
C
I
SW
D
OUT
OUT
×
×
V
---------
L
IN
ΔT
1
5
SS
capacitor. The EN pin
(Continued)
V
IN
= 3.3V
FIGURE 7. TRANSIENT RESPONSE
IN
/L, L is
(EQ. 1)
V
ISL97656
O
= 12V
f
s
= 1.2MHz
During the second cycle, the power FET turns off and the
Schottky diode is forward biased, (Figure 10). 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 clamp to a Schottky
diode above the output voltage. So the voltage drop across
the inductor is V
during the second cycle is shown in Equation 2:
ΔT2
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.
ΔI1
V
--------------- -
ΔI
--------- -
f
SW
V
D
OUT
L
I
IN
O
+
=
×
= 50mA to 300mA
V
=
ΔI2
V
---------
IN
ΔT2
=
L
1 D
------------ -
IN
f
SW
–
------------ -
1 D
C
=
+
IN
–
1
×
1 D
------------ -
0
f
V
------------------------------- -
SW
–
IN
FIGURE 8. BOOST CONVERTER
–
IN
×
ISL97656
L
V
V
------------------------------- -
- V
OUT
L
IN
OUT
–
L
V
OUT
. The change in inductor current
=
0
D
C
OUT
September 10, 2007
V
FN6439.0
(EQ. 2)
(EQ. 3)
OUT