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LT3504

Quad 40V/1A Step-Down Switching Regulator with 100% Duty Cycle Operation

Linear Technology 

LT3504

Applications Information

FB Resistor Network

The output voltage is programmed with a resistor divider

connected from the output and the FB pin. Choose the 1%

resistor according to:

R1 =

R2

•

⎛

⎝⎜

VOUT

0.8V

−

1⎞⎠⎟

A good value for R2 is 10kΩ, R2 should not exceed 20kΩ

to avoid bias current error.

At input voltages that exceed VIN(PS) the part will continue

to regulate the output voltage up to 40V. However the

part will skip pulses (see Figure 1) resulting in unwanted

harmonics, increased output voltage ripple, and increased

peak inductor current. Provided that the inductor does not

saturate and that the switch current remains below 2A,

operation above VIN(PS) is safe and will not damage the

part. For a more detailed discussion on minimum on-time

and pulse-skipping, refer to the Applications Information

section of the LT3505 data sheet.

Input Voltage Range

The input voltage range for LT3504 applications depends

on the output voltage and on the absolute maximum rat-

ing of the VIN pin.

The minimum input voltage to regulate the output gener-

ally has to be at least 400mV greater than the greatest

programmed output voltage. The only exception is when

the largest programmed output voltage is less than 2.8V.

In this case the minimum input voltage is 3.2V.

Avoid starting up the LT3504 at input voltages greater

than 36V, as the LT3504 must simultaneously conduct

maximum currents at high VIN. The maximum operating

junction temperature of 125°C may be exceeded due to

the high instantaneous power dissipation.

IL

0.5A/DIV

The absolute maximum input voltage of the LT3504 is

40V and the part will regulate output voltages as long

as the input voltage remains less than or equal to 40V.

However for constant-frequency operation (no pulse-

skipping) the maximum input voltage is determined by

the minimum on-time of the LT3504 and the programmed

switching frequency. The minimum on-time is the shortest

period of time that it takes the switch to turn on and off.

Therefore the maximum input voltage to operate without

pulse-skipping is:

VSW

10V/DIV

2µs/DIV

3504 F01a

Figure 1a: The LT3504 Operating in Constant-Frequency

Operation (Below VIN(PS)), VIN = 26.5V, VOUT = 3.3V,

fSW = 2MHz, tON(MIN) = 74ns at T = 25°C

VIN(PS) = [ (VOUT + VD)/(fSW • tON(MIN)) ] + VSW – VD

where:

IL

0.5A/DIV

• VIN(PS) is the maximum input voltage to operate in

constant frequency operation without skipping pulses.

• VOUT is the programmed output voltage

• VSW is the switch voltage drop, at IOUT = 1A, VSW = 0.4V

• VD is the catch diode forward voltage drop, for an ap-

propriately sized diode, VD = 0.4V

• fSW is the programmed switching frequency

VSW

10V/DIV

2µs/DIV

3504 F01b

Figure 1b.The LT3504 Operating in Pulse-Skipping Mode

(Above VIN(PS)), VIN = 27V, VOUT = 3.3V, fSW = 2MHz,

tON(MIN) = 74ns at T = 25°C

• tON(MIN) is the minimum on-time, worst-case over

temperature = 110ns (at T = 125°C)

3504fa

10

For more information www.linear.com/LT3504

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