Datasheet

LT3970 Series
15
3970fc
applicaTions inFormaTion
Be aware that while V
IN
is below 4.2V, the input current
may rise up to several hundred µA and the part may begin
to switch while the internal circuitry starts up. Figure 7
shows the startup behavior of a typical application with
different programmed V
IN(EN)
.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces
-
sively, a LT3970 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
LT3970 is absent. This may occur in battery charging ap
-
plications or in battery backup systems where a battery
or some other supply is diode ORed with the LT3970’
s
output. If the V
IN
pin is allowed to float and the EN pin
is held high (either by a logic signal or because it is tied
to V
IN
), then the LT3970’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. If the EN pin is
grounded, the SW pin current will drop to 0.7µA. However,
if the V
IN
pin is grounded while the output is held high,
regardless of EN, parasitic diodes inside the LT3970 can
pull current from the output through the SW pin and the
V
IN
pin. Figure 8 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
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 LT3970 Runs Only when the Input is Present
160
120
INPUT CURRENT (µA)
80
40
0
4
INPUT VOLTAGE (V)
3
OUTPUT VOLTAGE (V)
2
1
0
160
120
INPUT CURRENT (µA)
80
40
0
4
3
OUTPUT VOLTAGE (V)
2
1
0
3970 F07
V
IN(EN)
= 6V
R3 = 5M
R4 = 1M
V
IN(EN)
= 12V
R3 = 11M
R4 = 1M
0 1 2 3 4 5 6 7 8
INPUT VOLTAGE (V)
0 2 4 6 8 10 12 14
Figure 7. V
IN
Start-Up of Front Page Application with V
OUT
= 3.3V,
No-Load Current, and V
IN(EN)
programmed as in Figure 6.
BD
LT3970
BOOSTV
IN
EN
V
IN
V
OUT
BACKUP
3970 F08
SW
D4
MBRS140
FBGND
+
Figure 6. Enable
+
–
1V
SHDN
3970 F06
LT3970
EN
V
IN
V
IN
R3
R4