Datasheet

LT3988
13
3988f
applicaTions inForMaTion
The boost circuit also limits the minimum input voltage for
proper start-up. If the input voltage ramps slowly, or the
LT3988 turns on when the output is already in regulation,
the boost capacitor may not be fully charged. Because
the boost capacitor charges with the energy stored in the
inductor, the circuit will rely on some minimum load current
to get the boost circuit running properly. This minimum
load will depend on input and output voltages, and on the
arrangement of the boost circuit. The minimum load cur-
rent generally goes to zero once the circuit has started.
Figure 4 shows a plot of input voltage to start and to run
as a function of load current. Even without an output load
current, in many cases the discharged output capacitor
will present a load to the switcher that will allow it to start.
The boost current is generally small but can become sig-
nificant at high duty cycles. The required boost current is:
I
BOOST
=
V
OUT
V
IN






I
OUT
40






Figure 3. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output
Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current, and Boost Circuit
Figure 2. Generating the Boost Voltage
Converter with Backup Output Regulator
There is another situation to consider: systems where the
output will be held high when the input to the LT3988 is
absent. If the V
IN
pin is grounded while the output is held
high, then diodes inside the LT3988 can pull large currents
from the output through the SW and V
IN
pins. A Schottky
diode in series with the input to the LT3988, as shown in
Figure 3, will protect the LT3988 and the system from a
shorted or reversed input.
V
IN
V
OUT
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
3988 F02
C3
V
IN
SW
GND
BD BOOST
(2a)
V
IN
V
OUT
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
C3
V
IN
SW
GND
BD BOOST
(2b)
V
IN
V
OUT
V
BOOST
– V
SW
≅ V
IN3
MAX V
BOOST
≅ V
IN3
+ V
IN
MIN VALUE FOR V
IN3
= 3V
V
IN
SW
GND
BD BOOST
(2c)
V
IN3
> 3V
3988 F03
V
IN
D4
SW
GND
V
OUT
LT3988
LOAD CURRENT (mA)
0
5.0
5.5
800
3988 F04a
4.5
4.0
400200 600 1000
3.5
INPUT VOLTAGE (V)
T
A
= 25°C
TO START
TO RUN
LOAD CURRENT (mA)
0
6.6
7.0
800
3988 F04b
6.2
5.8
400200 600 1000
5.4
INPUT VOLTAGE (V)
T
A
= 25°C
TO START
TO RUN
Minimum Input Voltage, V
OUT
= 3.3V Minimum Input Voltage, V
OUT
= 5V