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LT3645

36V 500mA Step-Down Regulator and 200mA LDO

Linear Technology 

LT3645

APPLICATIONS INFORMATION

FB Resistor Networks

The output voltages are programmed with resistor dividers

between the outputs and the VFB and VFB2 pins. Choose

the resistors according to

R1" R2 t

©

ǻ

VOUT

0.8

– 1¹»º

R3

"

R4

t

©

ǻ

VOUT2

0.797

–

1¹»º

R2 and R4 should be 20k or less to avoid bias current

errors. In the step-down converter, an optional phase

lead capacitor of 22pf between VOUT and VFB reduces

light-load ripple.

voltages up to 55V, but once the input voltage exceeds

36V, the power switch will shut off and stop regulating

the output voltage until the input voltage falls below 36V.

Minimum On Time

The LT3645 will operate at the correct frequency while

the input voltage is below VIN(MAX). At input voltages

that exceed VIN(MAX), the LT3645 will still regulate the

output properly (up to 38.5V); however, the LT3645 will

skip pulses to regulate the output voltage resulting in

increased output voltage ripple.

Figure 1 illustrates switching waveforms for a LT3645

application with VOUT = 1.2V near VIN(MAX) = 21.3V.

Input Voltage Range

The maximum operating input voltage for the LT3645 is

36V. The minimum input voltage is determined by either

the LT3645’s minimum operating voltage of 3.6V or by

its maximum duty cycle. The duty cycle is the fraction of

time that the internal switch is on and is determined by

the input and output voltages:

DC = (VOUT + VD)/(VIN – VSW + VD)

where VD is the forward voltage drop of the catch diode

(~0.4V) and VSW is the voltage drop of the internal switch

(~0.4V at maximum load). This leads to a minimum input

voltage of:

VIN(MIN) = ((VOUT + VD)/DCMAX) – VD + VSW

with DCMAX = 0.83 for the LT3645.

The maximum input voltage is determined by the absolute

maximum ratings of the VIN and BOOST pins. For fixed

frequency operation, the maximum input voltage is de-

termined by the minimum duty cycle, which is:

VIN(MAX) = ((VOUT + VD)/DCMIN) – VD + VSW

with DCMIN = 0.075 for the LT3645.

Note that this is a restriction on the operating input voltage

for continuous mode operation. The circuit will continue

to regulate the output up until the overvoltage lockout

input voltage (38.5V). The part will tolerate transient input

SWITCH

VOLTAGE

10V/DIV

INDUCTOR

CURRENT

0.5A/DIV

VIN = 18V

VOUT = 1.2V

IOUT = 500mA

COUT = 10μF

L = 10μH

Figure 1.

3645 F01

As the input voltage is increased, the part is required

to switch for shorter periods of time. Delays associated

with turning off the power switch dictate the minimum on

time of the part. The minimum on time for the LT3645 is

100ns. Figure 2 illustrates the switching waveforms when

the input voltage is increased to VIN = 22V.

SWITCH

VOLTAGE

10V/DIV

INDUCTOR

CURRENT

0.5A/DIV

VIN = 22V

VOUT = 1.2V

IOUT = 500mA

COUT = 10μF

L = 10μH

Figure 2.

3645 F02

3645f

10

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