isl97656 Intersil Corporation, isl97656 Datasheet - Page 7

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

I
the rising rate of the output voltage. The regulator goes
through the start-up sequence as well after the EN pin is
pulled to HI.
Frequency Selection
The ISL97656 switching frequency can be user selected to
operate at either constant 640kHz or 1.25MHz. Connecting
FREQ pin to ground sets the PWM switching frequency to
640kHz. When connecting FREQ high or IN, the switching
frequency is set to 1.2MHz.
Shutdown Control
When the EN pin is pulled down, the ISL97656 is shutdown
reducing the supply current to <1µA.
Maximum Output Current
The MOSFET current limit is nominally 4.0A and guaranteed
3.0A. This restricts the maximum output current, I
based on Equation 7:
where:
ΔI
V
f
I
D = MOSFET turn-on ratio:
Table 1 gives typical maximum I
switching frequency and 10µH inductor.
L
D
S
L-AVG
DIODE
L
=
I
I
ΔI
= switching frequency, 600kHz or 1.2MHz
=
L
L(AVG)
=
L
I
= MOSFET current limit
1
L AVG
= inductor ripple current
(
V
------------------------------------------------------------------------------
–
V
=
IN
--------------------------------------------
V
IN
L
= Schottky diode forward voltage, typically, 0.6V
2.5
2.5
2.5
3.3
3.3
3.3
TABLE 1. TYPICAL MAXIMUM I
OUT
5
5
I
-------------
1 D
×
= average inductor current
×
OUT
(V)
)
–
[
(
+
(
V
V
(
V
+
O
1 2 ⁄
O
IN
V
+
+
DIODE
V
V
×
DIODE
DIODE
ΔI
L
)
)
) V
V
×
–
OUT
f
S
12
12
12
5
9
5
9
9
IN
7
(V)
]
OUT
values for 1.2MHz
OUT
VALUES
I
OMAX
1790
2370
1300
1970
1470
990
750
970
OMAX
(mA)
(EQ. 10)
(EQ. 7)
(EQ. 8)
(EQ. 9)
,
ISL97656
Cascaded MOSFET Application
A 24V N-Channel MOSFET is integrated in the boost
regulator. For the applications where the output voltage is
greater than 24V, an external cascaded MOSFET is needed
as shown in Figure 11. The voltage rating of the external
MOSFET should be greater than A
DC PATH BLOCK APPLICATION
Note that there is a DC path in the boost converter from the
input to the output through the inductor and diode, hence the
input voltage will be seen at output with a forward voltage
drop of diode before the part is enabled. If this voltage is not
desired, the following circuit can be inserted between input
and inductor to disconnect the DC path when the part is
disabled.
FIGURE 12. CIRCUIT TO DISCONNECT THE DC PATH OF
TO INDUCTOR
FIGURE 11. CASCADED MOSFET TOPOLOGY FOR HIGH
V
IN
EN
BOOST CONVERTER
OUTPUT VOLTAGE APPLICATIONS
INTERSIL
ISL97656
LX
IN
FB
.
September 10, 2007
INPUT
FN6439.0
A
IN