M
0.075A to 1.5A, Programmable
Current-Limit Switch
8
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Detailed Description
The MAX4766 is a current-limit switch that operates
from a 2.25V to 4.5V input voltage range and has a pro-
grammable current limit from 0.075A to 1.5A with a
±10% accurate current limit. The device features an
adjustable blanking time, an adjustable soft-start time,
and an output indicator,
FLAG,
that notifies the proces-
sor when a fault condition is present (Figure 1). The
MAX4766 is available in latch-off, autoretry, and contin-
uous current-limit versions (see the
Selector Guide
).
Programmable Current Limit
A resistor from SETI to GND programs the current limit
for the switch (see the
Setting the Current Limit
section).
The MAX4766 increases the pass transistor’s gate drive
when the programmed current-limit value is reached. If
the load current exceeds the current limit for longer than
the t
BLANK
period, the switch either goes into autoretry,
latches off, or stays in the continuous mode until the
thermal limit is reached.
Programmable Soft-Start
The MAX4766 features an adjustable soft-start time to
control the output voltage ramp rate, thereby limiting the
inrush surge current. An external capacitor from SETS to
GND sets the soft-start time. The value of the capacitor
directly determines the typical soft-start time. Use the
following formula to determine the soft-start time:
Leave SETS floating to choose 1ms minimum preset
soft-start time.
Programmable Blanking Time
(MAX4766A/MAX4766B)
The adjustable fault-blanking time feature of the
MAX4766 ensures that no fault signal asserts during
power-up or when there is a momentary short caused
by hot-swapping a capacitive load. An external capaci-
tor from SETB to GND sets the blanking time. When a
short or load transient causes the device to enter current
limit, an internal counter monitors the duration of the
fault after the soft-start time elapses. For the MAX4766B,
FLAG
signal asserts low when load faults exceed the
programmed blanking time (t
BLANK
). If the fault condi-
tion is removed before the programmed blanking time
period has elapsed, the timer resets and the
FLAG
sig-
nal does not assert (see Figures 2a and 2b). Only cur-
rent-limit and short-circuit faults are blanked.
Soft Start ms
C
nF
1
ms
1
SETS
=
×
(
)
Figure 2a. Latch-Off Fault Blanking (Toggling ON)
LOAD
CURRENT
SWITCH
STATUS
ON
VOLTAGE
t
BLANK
t
BLANK
OFF
ON
FLAG
STATUS
Figure 2b. Latch-Off Fault Blanking (Cycling IN Below UVLO)
LOAD
CURRENT
SWITCH
STATUS
V
IN
UNLO
OFF
ON
t
BLANK
t
BLANK
FLAG PIN
STATUS
C
SETB
(μF)
TYPICAL BLANKING TIME
(ms)
1
8
19
35
50
65
80
100
115
125
150
Open
0.01
0.025
0.05
0.075
0.1
0.125
0.150
0.175
0.2
0.25
Table 1. Blanking Time vs. Capacitor Values