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CM3708A
3A L
OW
-N
OISE
PWM S
TEP
-D
OWN
R
EGULATOR
FUNCTIONAL DESCRIPTION
The CM3708A step-down, pulse-width-modulated (PWM),
DC-DC converter has an adjustable output range from VREF
to the input voltage (VIN). An internal synchronous rectifier
improves efficiency and eliminates an external Schottky diode.
Fixed-frequency operation enables easy post-filtering, thereby
providing excellent noise characteristics. As a result, the
CM3708A is an ideal choice for many small wireless systems.
VREF
The reference voltage could be ranged from 0V to VIN.
OUPUTS
The output voltage pins (LX1, LX2) are tied to the RF power
amp, via an external inductor. Output voltage is determined by
the VREF inputs so it can be from 0V to VIN.
Internal Power Switches:
CM3708A has been integrated 2 Power MOSFETs whose
RDS
(ON)
is around 100m Ohm each.
Low Noise Current Limit:
CM3708A’s current limit is a low noise current limit. It increase
the system loop noise immunity while it is doing the current
limit protection for the system. The current limit is around 6A
for the normal 3A operation.
APPLICATION CIRCUIT
INPUTS
The input voltage reference pin, VREF determine the output
voltages (LX1, LX2). If a specific voltage is forced at the VREF
pin, the output voltage follows the voltage at the VREF pin.
OTHER SUPPLY VOLTAGES
Several inputs are provided for the supply voltages: PVIN and
VIN.
The PVIN provide the power supply to the power MOSFETs.
VIN provides the voltage supply to the logic section and internal
error amplifiers.
FEEDBACK
The VFB pin is an input that can be used for closed loop
compensation. This input is derived from the voltage output.
AGND pin is a contact node of internal resistor divider for
remote sense.
PVIN
VIN
PVIN
PVIN
R1
5R1
R3
1k
2
1
VREF
VIN
R2
1
200k
2
C6
100uf/6.3V
C2
104
J2
DC-INPUT
1
2
U1
CM3708A
2
1
3
13
14
15
16
4
5
6
7
8
9
10
11
12
PVIN1
VIN1
LX1
PGND2
LX2
PVIN2
VIN2
PGND1
AGND
SD
VREF
AGSEN
AGND
NC
VFB
AGND
C9
104
C1
220uf/6.3V
C8
104
C12
102
C7
104
L1
3.3uH
C11
104
J1
Vout
1
2
C3
10uf/6.3V
C10
100uf/6.3V
C4
104
C5
470uf/6.3V
2004/12/13 Champion Microelectronic Corporation
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