LT3973 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT3973
Linear Technology
'LT3973' PDF : 24 Pages View PDF
LT3973/LT3973-3.3/LT3973-5
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.
VUVLO
=
R3 + R4
R4
•
1.19V
where switching should not start until VIN is above VUVLO.
Note that due to the comparator’s hysteresis, switching
will not stop until the input falls slightly below VUVLO.
Undervoltage lockout is functional only when VUVLO is
greater than 5.5V.
9
FRONT PAGE APPLICATION
8
7
VIN
6
5
VOUT
4
3
2
1
0
TIME
3973 F06
Figure 6. VIN to VOUT Performance
VIN
VIN
LT3973
R3
1.19V +
EN/UVLO
–
R4
SHDN
3973 F07
Figure 7. Undervoltage Lockout
D4
BD
VIN
VIN
BOOST
LT3973
EN/UVLO SW
VOUT
Enable and Undervoltage Lockout
GND
FB
+
BACKUP
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 VIN is above 4.2V. If VIN
is lower than 4.2V, tie EN/UVLO pin to GND to place the
part in shutdown.
3973 F08
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
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:
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 VIN
pin is allowed to float and the EN/UVLO pin is held high
(either by a logic signal or because it is tied to VIN), then
the LT3973’s internal circuitry will pull its quiescent current
through its SW pin. This is fine if the system can tolerate
3973fb
For more information www.linear.com/LT3973
17
Share Link: 