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3433EFE

High Voltage Step-Up/Step-Down DC/DC Converter

Linear Technology 

LT3433

APPLICATIO S I FOR ATIO

Design Example

4V-60V to 5V DC/DC converter (the application on the

front page of this data sheet), load capability for TA = 85°C.

Application Specific

Constants:

LT3433 W/C Constants:

VIN = 4V

VOUT = 5V

L = 100µH

RL = 0.28Ω

VF1 = 0.45V

VF2 = 0.4V

RCESR = 0.01Ω

IMAX = 0.55A

RSWH = 1.2Ω

RSWL = 1Ω

fO = 190kHz

∆BST = 0.05

∆OUT = 0.05

IVIN = 600µA

IBIAS = 800µA

The LT3433 operates in bridged mode with VIN = 4V, so the

relations used are:

DC = [VOUT + VF1 + VF2 – ISW • (RL + RESR)]/[VIN –

ISW • (RSWH + RSWL + 2RL + RESR) + VOUT + VF1 +

VF2]

∆I = (VOUT + VF1 + VF2 - ISW • RL) • (1 – DC)/(L • fO)

IOUT(MAX) = ISW • [1 – DC • (1 + ∆BST + ∆OUT)] – IBIAS

Iteration procedure for DC:

(1) Set initial seed value for ∆I (this example will set

∆I = 0).

(2) Using seed value for ∆I, determine ISW (ISW = 0.55 –

0 = 0.55).

(3) Use calculated ISW and above design constants to

solve the DC relation (DC = 0.683).

(4) Use calculated DC to solve the ∆I relation (yields ∆I =

0.0949).

(5) If calculated ∆I is equal to the seed value, stop.

Otherwise, use calculated ∆I as new seed value and

repeat (2) through (4).

ITERATION #

1

2

3

SEED ∆I

0

0.095

0.098

CALCULATED VALUES

ISW

DC

∆I

0.55

0.683

0.095

0.503

0.674

0.098

0.501

0.674

0.098

After iteration, DC = 0.674 and ∆I = 0.098.

Use iteration result for DC and above design constants to

solve the IOUT(MAX) relation:

IOUT(MAX) = 0.501 • [1 – 0.674 • (1 + 0.05 + 0.05)] –

800µA

IOUT(MAX) = 129mA

Increased Output Voltages

The LT3433 can be used in converter applications with

output voltages from 3.3V through 20V, but as converter

output voltages increase, output current and duty cycle

limitations prevent operation with VIN at the extreme low

end of the LT3433 operational range. When a converter

operates as a buck/boost, the output current becomes

discontinuous, which reduces output current capability by

roughly a factor of 1 – DC, where DC = duty cycle. As such,

the output current requirement dictates a minimum input

voltage where output regulation can be maintained.

Typical Minimum Input Voltage as a Function of

Output Voltage and Required Load Current

24

20

200mA

16

175mA

12

125mA

8

150mA

4

4

8

12

16

20

VOUT (V)

3433 AI03

3433f

12

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