MAX746 Maxim, MAX746 Datasheet - Page 8

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

High-Efficiency, PWM, Step-Down,
N-Channel DC-DC Controller
Figure 1b. 3.3V Standard Application Circuit (9.9W)
with light loads, the PWM comparator trips quickly and the
idle-mode comparator sets the EXT on-time.
Traditional PWM converters continue to switch on every
cycle, even when the inductor current is discontinuous
due to smaller loads, decreasing light-load efficiency.
In contrast, the MAX746’s idle-mode comparator increas-
es the switch on-time, allowing more energy to be trans-
ferred per cycle. Since fewer cycles are required, the
switching frequency is reduced, resulting in minimal
switching losses and increased efficiency.
The light-load output noise spectrum widens due to the
variable switching frequency in idle-mode, but output
ripple remains low. Using the Typical Operating Circuit,
with a 9V input and a 125mA load current, output ripple
is less than 40mV.
The MAX746 contains all the control circuitry required
to provide a regulated charge-pump voltage 5V
above V+ for high-side driving N-channel logic FETs.
The charge pump operates with a nominal 50kHz fre-
8
_______________________________________________________________________________________
*
4.5V TO 6V
SUMIDA CDR125 22 H SURFACE-MOUNT INDUCTOR
R2
R1
V IN
0.1 F
C3
0.1 F
1 F
C5
C6
11
2
3
4
5
LBI
SS
REF
SHDN
AGND
C2
100 F
MAX746
GND
V+
15
16
HIGH
Charge Pump
OUT
AV+
EXT
LB0
CC
CP
CS
FB
1N914
14
13
8
10
12
7
9
6
1
D2
C8
0.1 F
1N914
C7
2nF
D3
1N914
C9
1 F
D5
R4
20k (1%)
13k (1%)
1N914
C10
0.1 F
D6
quency. When the voltage at HIGH exceeds AV+ by
5V, the charge-pump oscillator is inhibited (Figure 2).
When the voltage at HIGH is less than 4.3V below V+,
undervoltage lockout occurs. Use the voltage tripler
(Figure 3b) when V+ ≤ 6V; otherwise, use the voltage
doubler (Figure 3a).
The MAX746 draws its highest current at power-up. If
the power source to the MAX746 cannot provide this
initial elevated current, the circuit may not function cor-
rectly. For example, after prolonged use the increased
series resistance of a battery may prevent it from pro-
viding adequate initial surge currents when the
MAX746 is brought out of shutdown. Using soft-start
(SS) minimizes the possibility of overloading the incom-
ing supply at power-up by gradually increasing the
peak current limit. Connect an external capacitor from
SS to AGND to reduce the initial peak currents drawn
from the supply.
R5
D4
1N5817
Si9410DY
NSQ03A03
Q1
Soft-Start and Current Limiting
C11
1 F
D1
N
R3
100k
R
40m
SENSE
22 H
L1
*
C4
0.1 F
AT 3A
C3
660 F
3.3V