Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
Copyright © 2000
Rev. 1.1c, 2005-03-03
WWW.Microsemi .COM
LX1732
Hi
h Current PFM Boost Converter
INTEGRATED PRODUCTS
APPLICATION NOTE
F
UNCTIONAL
D
ESCRIPTION
The LX1732 implements a constant on-time and off-
time PFM architecture that can deliver a 5.0V output and
over 150mA of continuous output current. The free-
running oscillator and logic-circuit initiate the internal
MOSFET switching cycle during power-up via the VC pin
(see block diagram in Figure 2)). The current passing
through the LX pin is detected by the I-sense circuit and
compared with the feedback voltage status via the logic
circuit. The internal limit for the peak current is set at
1000mA (max). The MOSFET switch remains on for the
on-time duration or until the I-sense detects the maximum
peak current, or when the feedback threshold voltage is
detected at the FB pin. The feedback threshold voltage is
set by an external resistor divider network and compared
with the internal reference voltage. The LX1732
implements a non-load isolated shutdown mode using an
input-voltage referenced signal level via the SHDN pin.
Connect this pin to the input supply rail if the shutdown
feature is not required. The LX1732 features a low-battery
detection circuit that allows for sensing of the input voltage
supply. If the supply voltage at LBI drops below the
internal reference voltage, the internal MOSFET (open-
drain) sinks current to GND.
O
UTPUT
V
OLTAGE
P
ROGRAMMING
Resistors R1 and R2 of Figure 1 program the output
voltage. An optional 1000pF capacitor is recommended
across R1 to improve the transient response and reduce
output voltage ripple (see Figure 7). The value of R2 should
be less than 250KΩ. The value of R1 can be determined
using the following equation where VREF is found in the
ELECTRICAL CHARACTERISTICS
T
ABLE
:
1
V
V
R2R1
REF
OUT
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
D
ESIGN
E
XAMPLE
:
Let R2 equal 100K and the required VOUT equal to
5.0V.
316.6KΩ 1100KR1
1.20
5V
=−=
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
D
IODE
S
ELECTION
A Schottky diode is recommended for use with the
LX1732 because it provides fast switching and superior
reverse recovery performance. The Microsemi UPS5817
(20V @ 1A) makes an effective choice for most
applications.
L
OW
B
ATTERY
D
ETECTION
P
ROGRAMMING
Program the Low Battery Detect voltage threshold by
selecting values for resistors R3 and R4 (see Figure 1) using
the formula below. Use a value of less than 250KΩ for R4 to
minimize threshold error due to the internal comparator’s
offset current. The value of R3 can be determined using the
following equation.
1
V
V
R4R3
REF
BATT LOW
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
The LBO pin’s open drain output requires a pull up resistor
(i.e., 100K typ.) to drive external CMOS logic circuits.
Connect the LBI pin to ground and omit resistors R3 and R4
when the Low Battery Detect function is not implemented.
D
ESIGN
E
XAMPLE
:
Let R3 equal 100K and the required LB threshold equal
to 2.5V.
316KΩ 1R100R3
0.6V
2.5V
=−=
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
C
APACITOR
S
ELECTION
To minimize ripple voltage, output capacitors in the range
of 10uF to 100uF with a low series resistance (ESR) are
recommended. Multi-layer ceramic capacitors with X5R or
X7R dielectric make an effective choice because they feature
small size, very low ESR, a temperature stable dielectric, and
can be connected in parallel to increase capacitance. Other
low ESR capacitors such as solid tantalum, specialty
polymer, or organic semiconductor, make effective choices
provided that the capacitor is properly rated for the output
voltage and ripple current. Finally, choose an input capacitor
of sufficient size to effectively decouple the input voltage
source impedance (e.g., C
IN
> 47µF).
L
AYOUT
C
ONSIDERATIONS
The high peak currents and switching frequencies present
in DC/DC converter applications require careful attention to
device layout for optimal performance. Basic design rules
include: (1) maintaining wide traces for power components
(e.g., width > 50mils); (2) place C
IN
, C
OUT
, the Schottky
diode, and the inductor close to the LX1732; (3) minimizing
trace capacitance by reducing the etch area connecting the
SW pin to the inductor; and (4) minimizing the etch length to
the LBI and FB pins to reduce noise coupling into these high
impedance sense inputs. Other considerations include
placing a 0.1uF capacitor between the LX1732 VOUT pin
and GND pin to reduce high frequency noise and decoupling
the VIN pin using a 0.1uF capacitor.
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
S
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