MAX746 Maxim, MAX746 Datasheet - Page 11

• Manufacturer Part Number: MAX746
• Description: High-Efficiency / PWM / Step-Down / N-Channel DC-DC Controller
• Manufacturer: Maxim

The MAX746’s dual-mode output voltage can be set
to 5V by grounding FB, or it can be adjusted from
2V to 14V using external resistors R4 and R5 config-
ured as shown in Figure 6. Select feedback resistor
R4 in the 10kΩ to 60kΩ range. R5 is given by:
The MAX746 is designed to use either internal or exter-
nal feedback mode, but should not be toggled between
Figure 3. Charge-Pump Configurations
__________________Design Procedure
T
OSCILLATOR
CLK
100kHz
T FLIP-
FLOP
MAX746
MAX746
GND
GND
V
V
V+
V+
IN
IN
R5 = (R4)
15
16
15
16
Q
______________________________________________________________________________________
AV+
HIGH
HIGH
Setting the Output Voltage
5V
CP
CP
1N914
14
13
D2
14
13
(
_______ – 1
V
C8
0.1 F
1N914
0.1 F
1N914
OUT
2V
D3
D2
C8
3a. CHARGE-PUMP
VOLTAGE DOUBLER
3b. CHARGE-PUMP
VOLTAGE TRIPLER
1N5817
High-Efficiency, PWM, Step-Down,
D4
C9
1 F
1N914
1N914
D3
D5
)
C10
0.1 F
1N5817
1N914
D4
D6
C9
1 F
C11
1 F
N-Channel DC-DC Controller
the two modes while operating. If two different output
voltages are required, use external feedback mode
with a resistor network similar to the 3.3V/5V adjustable
output circuit shown in Figure 7.
To select the sense-resistor value (R
approximate the peak current assuming I
(1.1) (I
rent. Once all component values have been deter-
mined, the actual peak current is given by:
Figure 4. Peak Current Limit vs. Soft-Start Voltage
Figure 5. Input Voltage Monitor Circuit
I
LOAD
PK
= I
R2
R1
), where I
LOAD
3
2
1
0
0
2
V+ - V
LBI
+
vs. SOFT-START VOLTAGE
CS
PEAK CURRENT LIMIT
(
= 150mV
1
SOFT-START VOLTAGE (V)
LOAD
MAX746
___________
(2L) (f
16
15
GND
V
V+
IN
V
OUT
R
is the maximum load cur-
2
SENSE
OSC
LBO
R
Selecting R
= 50m
SENSE
)
)(
1
3
= 100m
TO V
R
V
1– _______
2
REF
= R1
OUT
= 2.0V
V
R3
100k
LOW-BATTERY
OR V
SENSE
V
(
OUT
IN
4
OUTPUT
V
V
TRIP
REF
IN
)
-1
SENSE
)
), first
PK
11
is