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LT3645

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 VIN 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 500μs internal soft-start for the

buck converter and a 500μs soft-start for the LDO regula-

tor. Both soft-starts are reset if the EN/UVLO pin is low, if

VIN drops below 3.4V (undervoltage), if VIN exceeds 36V

(overvoltage), or when the die temperature exceeds 160°C

Figure 4.

(4a) Typical Minimum Input Voltage,

VOUT = 5V

(4b) Typical Minimum Input Voltage,

VOUT = 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 OR’d with

the LT3645’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 LT3645’s internal

circuitry will draw its quiescent current through its SW

pin. This is ﬁne 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 VIN pin is grounded while the output

is held high, then parasitic diodes inside the LT3645 can

OUTPUT CURRENT (mA)

5.0

5.5

6.5

6.0

7.5

7.0

8.0

100

3645 F04a

1000

VIN TO RUN

VIN TO START

OUTPUT CURRENT (mA)

3.0

4.0

3.5

5.0

4.5

6.0

5.5

6.5

100

3645 F04b

1000

VIN TO RUN

VIN TO START

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