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LT3957AEUHE

Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch

Linear Technology 

LT3957A

APPLICATIONS INFORMATION

ΔISW increases output current capability, but requires

large inductances and reduces the current loop gain (the

converter will approach voltage mode). Accepting larger

values of ΔISW provides fast transient response and

allows the use of low inductances, but results in higher

input current ripple and greater core losses, and reduces

output current capability.

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the inductor,

the inductor value of the boost converter can be determined

using the following equation:

L

=

VIN(MIN)

ΔISW • ƒ

• DMAX

The peak inductor current is the switch current limit (5.9A

typical), and the RMS inductor current is approximately

equal to IL(MAX). The user should choose the inductors

having sufficient saturation and RMS current ratings.

Boost Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with low

forward drop and low reverse leakage is desirable. The

peak reverse voltage that the diode must withstand is

equal to the regulator output voltage plus any additional

ringing across its anode-to-cathode during the on-time.

The average forward current in normal operation is equal

to the output current.

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT by a safety margin (a 10V

safety margin is usually sufficient).

The power dissipated by the diode is:

PD = IO(MAX) • VD

where VD is diode’s forward voltage drop, and the diode

junction temperature is:

TJ = TA + PD • RθJA

The RθJA to be used in this equation normally includes the

RθJC for the device plus the thermal resistance from the

board to the ambient temperature in the enclosure. TJ must

not exceed the diode maximum junction temperature rating.

14

Boost Converter: Output Capacitor Selection

Contributions of ESR (equivalent series resistance), ESL

(equivalent series inductance) and the bulk capacitance

must be considered when choosing the correct output

capacitors for a given output ripple voltage. The effect of

these three parameters (ESR, ESL and bulk C) on the output

voltage ripple waveform for a typical boost converter is

illustrated in Figure 5.

The choice of component(s) begins with the maximum

acceptable ripple voltage (expressed as a percentage of

the output voltage), and how this ripple should be divided

between the ESR step ΔVESR and the charging/discharg-

ing ΔVCOUT. For the purpose of simplicity, we will choose

2% for the maximum output ripple, to be divided equally

between ΔVESR and ΔVCOUT. This percentage ripple will

change, depending on the requirements of the application,

and the following equations can easily be modified. For a

1% contribution to the total ripple voltage, the ESR of the

output capacitor can be determined using the following

equation:

ESRCOUT

≤

0.01• VOUT

ID(PE AK )

For the bulk C component, which also contributes 1% to

the total ripple:

COUT

≥

IO(M AX )

0.01• VOUT

•

ƒ

tON

VOUT

(AC)

tOFF

)VCOUT

)VESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

3957A F05

Figure 5. The Output Ripple Waveform of a Boost Converter

The output capacitor in a boost regulator experiences high

RMS ripple currents, as shown in Figure 5. The RMS ripple

current rating of the output capacitor can be determined

using the following equation:

IRMS(COUT) ≥ IO(MAX) •

DMAX

1− DM AX

3957afa

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