MAX746 Maxim, MAX746 Datasheet - Page 9

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

The steady-state SS pin voltage is typically 3.8V. On
power-up, SS sources 1µA until its voltage reaches
3.8V. The current-limit comparator inhibits EXT switch-
ing until the SS voltage reaches 1.8V. The peak current
limit is set by:
where V
sense amplifier inputs. Figure 4 shows how the SS peak
current limit increases as the voltage on SS rises for two
R
Undervoltage lockout inhibits operation of EXT until the
charge pump is capable of generating a voltage greater
than 4.3V above the supply voltage (Figure 2). When
the undervoltage-lockout comparator detects an under-
voltage condition, the switching action at EXT is halted.
Figure 1c. 5V Standard Application Circuit (7.5W)
SENSE
LIMIT
values.
I
PK
is the differential voltage across the current-
= _________ = ___________
R
V
_______________________________________________________________________________________
SENSE
LIMIT
6V TO 15V
*
**
SEE TABLE 2 FOR DIODE SELECTION.
V IN
SUMIDA CDR125 SURFACE-MOUNT INDUCTOR.
Undervoltage Lockout
R2
R1
150mV (typ)
R
0.1 F
C3
0.1 F
1 F
C5
C6
SENSE
High-Efficiency, PWM, Step-Down,
11
2
3
4
6
5
LBI
SS
REF
FB
SHDN
AGND
C2
47 F
MAX746
GND
V+
N-Channel DC-DC Controller
15
16
HIGH
OUT
AV+
EXT
LB0
CC
CP
CS
1N914
14
13
8
10
12
7
9
1
D2
When SHDN is high, the MAX746 is shut down. In this
mode, the internal biasing circuitry (including EXT) is
turned off, V
drops to 1.4µA (20µA max). This excludes external
component leakage, which may add several
microamps to the shutdown supply current for the
entire circuit. SHDN is a logic input. Connect SHDN to
GND for normal operation.
The MAX746 provides a low-battery comparator that
compares the voltage on LBI to the reference voltage.
LBO, an open-drain output, goes low when the LBI volt-
age is below V
shown in the Input Voltage Monitor Circuit (Figure 5),
to set the trip voltage (V
this circuit, LBO goes low when V+ ≤ V
impedance in shutdown mode.
1N5817
*
D4
C7
1nF
C4
0.1 F
C8
0.1 F
1N914
D3
Si9410DY
NSQ03A03
*
OUT
Q1
REF
C9
4.7 F
D1
drops to 0V, and the supply current
. Use a resistor-divider network, as
R3
100k
N
R
75m
SENSE
L1
82 H
TRIP
**
Low-Battery Detector
) at the desired level. In
AT 1.5A
5V
C1
220 F
Shutdown Mode
TRIP
. LBO is high
9