LT3645
13
3645f
APPLICATIONS INFORMATION
boost circuit. If the input voltage is ramped slowly, or if
the LT3645 is turned on with the EN/UVLO pin when the
output is already in regulation, then the boost capacitor
might not be fully charged. Because the boost capacitor is
charged 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 generally
goes to zero once the circuit has started. The worst case
situation is when V
IN
is ramping very slowly. Figure 4a
shows the minimum input voltage needed to start a 5V
application versus output current. Figure 4b shows the
minimum input voltage needed to start a 3.3V application
versus output current.
Soft-Start
The LT3645 includes a 500約 internal soft-start for the
buck converter and a 500約 soft-start for the LDO regula-
tor. Both soft-starts are reset if the EN/UVLO pin is low, if
V
IN
drops below 3.4V (undervoltage), if V
IN
exceeds 36V
(overvoltage), or when the die temperature exceeds 160癈
Figure 4.
(4a) Typical Minimum Input Voltage,
V
OUT
= 5V
(4b) Typical Minimum Input Voltage,
V
OUT
= 3.3V
(thermal shutdown). The soft-start for the LDO can also be
reset by pulling the EN2 pin low. The soft-start functions
act to reduce the maximum input current during startup.
Soft-start can not be disabled in the LT3645.
Reversed Input Protection
In some systems, the output will be held high when the
input to the LT3645 is absent. This may occur in bat-
tery charging applications or in battery backup systems
where a battery or some other supply is diode ORd with
the LT3645s output. If the V
IN
pin is allowed to oat and
the EN/UVLO pin is held high (either by a logic signal
or because it is tied to V
IN
), then the LT3645s internal
circuitry will draw its quiescent current through its SW
pin. This is ne if the system can tolerate a few mA in this
state. You can reduce this current by grounding the EN/
UVLO pin, then the SW pin current will drop to essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3645 can
OUTPUT CURRENT (mA)
1
5.0
5.5
6.5
6.0
7.5
7.0
8.0
10
100
3645 F04a
1000
V
IN
TO RUN
V
IN
TO START
OUTPUT CURRENT (mA)
1
3.0
4.0
3.5
5.0
4.5
6.0
5.5
6.5
10
100
3645 F04b
1000
V
IN
TO RUN
V
IN
TO START