Datasheet

LT3973/LT3973-3.3/LT3973-5
17
3973fa
applicaTions inFormaTion
During dropout conditions when the output is below regula-
tion, the output ripple may increase. At very high loads, this
ripple can increase to approximately 200mV for the front
page application. If lower output ripple is desired during
such conditions, a larger output capacitor can be used.
In order to not exceed the maximum voltage rating, tie
the OUT pin to GND for programmed outputs greater than
14V. Note that this will result in degraded start-up and
dropout performance.
TIME
VOLTAGE (V)
6
7
9
8
5
3973 F06
4
3
2
1
0
FRONT PAGE APPLICATION
V
IN
V
OUT
Figure 6. V
IN
to V
OUT
Performance
V
UVLO
=
R3 +R4
R4
• 1.19V
where switching should not start until V
IN
is above V
UVLO
.
Note that due to the comparator’s hysteresis, switching
will not stop until the input falls slightly below V
UVLO
.
Undervoltage lockout is functional only when V
UVLO
is
greater than 5.5V.
+
–
1.19V
SHDN
3973 F07
LT3973
EN/UVLO
V
IN
V
IN
R3
R4
Figure 7. Undervoltage Lockout
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively,
a LT3973 buck regulator will tolerate a shorted output. There
is another situation to consider in systems where the output
will be held high when the input to the LT3973 is absent.
This may occur in battery charging applications or in battery
backup systems where a battery or some other supply is
diode ORed with the LT3973’s output. If the V
IN
pin is allowed
to float and the EN/UVLO pin is held high (either by a logic
signal or because it is tied to V
IN
), then the LT3973’s internal
circuitry will pull its quiescent current through its SW pin.
This is fine if the system can tolerate a few µA in this state.
Figure 8. Diode D4 Prevents a Shorted Input from Discharging a
Backup Battery Tied to the Output. It Also Protects the Circuit from
a Reversed Input. The LT3973 Runs Only When the Input Is Present
BD
LT3973
BOOSTV
IN
EN/UVLO
V
IN
V
OUT
BACKUP
3973 F08
SW
D4
FBGND
+
Enable and Undervoltage Lockout
The LT3973 is in shutdown when the EN/UVLO pin is low
and active when the pin is high. The rising threshold of the
EN/UVLO comparator is 1.19V, with a 30mV hysteresis.
This threshold is accurate when V
IN
is above 4.2V. If V
IN
is lower than 4.2V, tie EN/UVLO pin to GND to place the
part in shutdown.
Figure 7 shows how to add undervoltage lockout (UVLO)
to the LT3973. 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. The UVLO threshold can
be adjusted by setting the values R3 and R4 such that they
satisfy the following equation: