Datasheet

LT3508
15
3508fd
Shutdown and Undervoltage Lockout
Figure 7 shows how to add undervoltage lockout (UVLO)
to the LT3508. Typically, UVLO is used in situations where
the input supply is current limited, or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current
limit or latch low under low source voltage conditions.
UVLO prevents the regulator from operating at source
voltages where the problems might occur.
An internal comparator will force the part into shutdown
below the minimum V
IN1
of 3.7V. This feature can be used to
prevent excessive discharge of battery-operated systems.
is much lower than the switching frequency. A phase-lead
capacitor (C
PL
) across the feedback divider may improve
the transient response.
If an adjustable UVLO threshold is required, the SHDN
pin can be used. The threshold voltage of the SHDN pin
comparator is 2.63V. Current hysteresis is added above the
SHDN threshold. This can be used to set voltage hysteresis
of the UVLO using the following:
R3 =
V
H
–V
L
7.5µA
R4 =
2.63V
V
L
– 2.63V
R3
–8µA
Example: switching should not start until the input is above
4.75V and is to stop if the input falls below 4V.
V
H
= 4.75V, V
L
= 4.0V
R3 =
4.75V – 4V
7.5µA
= 100k
R4 =
2.63V
4V – 2.63V
100k
–8µA
= 461k
Keep the connection from the resistor to the SHDN pin
short and make sure the interplane or surface capacitance
to switching nodes is minimized. If high resistor values are
used, the SHDN pin should be bypassed with a 1nF capaci-
tor to prevent coupling problems from the switch node.
Soft-Start
The output of the LT3508 regulates to the lowest voltage
present at either the TRACK/SS pin or an internal 0.8V
reference. A capacitor from the TRACK/SS pin to ground
is charged by an internal 1.2A current source resulting
in a linear output ramp from 0V to the regulated output
whose duration is given by:
t
RAMP
=
C
SS
• 0.8V
1.2µA
At power up, internal open-collector outputs discharge
both TRACK/SS pins. The pins clamp at 1.3V.
APPLICATIONS INFORMATION
–
+
0.8V
V
SW
V
C
LT3508
GND
3508 F06
R1
OUTPUT
ESR
C
F
C
C
R
C
2M
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 2.5S
g
m
=
300µS
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 6. Model for Loop Response
Figure 7. Undervoltage Lockout
2.6V
7.5µA
8µA
R3
R4C1
SHDN
LT3508
V
IN
TRACK/SS
V
C
3508 F07
–
+