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+ V – V

OUT

– V

OUT

+ V – V

OUT

SAT

OUT

≈

LOAD

1 – D

V – V

SAT

100,000

•

LOAD

∆I

IND

1 – D

LOAD

∆I

IND

1 – D

+ V

OUT

– V

OUT

SAT

LOAD

∆I

IND

1 – D

LOAD

• •

1 – D

50 (1 – D)

0.25

Ω

(

1. Output Voltage Section

2. Inductor Selection (L)

TL3577

100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR

SLVS633 – OCTOBER 2006

Table 1. Step-up Regulator Formulas(1)

Duty cycle

Average inductor current

IIND(AVG)

Inductor current ripple

∆I

IND

Peak inductor current

IIND(PK)

Peak switch current

ISW(PK)

Switch voltage when off

VSW(OFF)

Diode reverse voltage

Average diode current

ID(AVG)

Peak diode current

ID(PK)

Power dissipation

(1)

VF = forward-biased diode voltage, ILOAD = output load

First, determine if the TL3577 can provide these values of VOUT and ILOAD(max) when operating with the minimum

value of VIN. The upper limits for VOUT and ILOAD(max) are given by the following equations.

VOUT ≤ 60 V and

VOUT ≤ 10 × VIN

ILOAD(max) ≤ (2.1 A × VIN(min))/VOUT

These limits must be greater than or equal to the values specified in this application.

Resistors R1 and R2 are used to select the desired output voltage. These resistors form a voltage divider and

present a portion of the output voltage to the error amplifier, which compares it to an internal 1.23-V reference.

Select R1 and R2 such that:

R1/R2 = (VOUT/1.23 V) – 1

A. Preliminary Calculations

To select the inductor, the calculation of the following three parameters is necessary:

Dmax, the maximum switch duty cycle (0 ≤ D ≤ 0.9):

Dmax = VOUT + VF – VIN(min)/VOUT + VF – 0.6 V

where, typically, VF = 0.5 V for Schottky diodes and VF = 0.8 V for fast-recovery diodes.

• T, the product of volts • time that charges the inductor:

• T = D

max × (VIN(min) – 0.6V)10

6/100,000 Hz (V

µs)

IIND,DC, the average inductor current under full load:

IIND,DC = (1.05 × ILOAD(max))/(1 – Dmax)

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