LT3757 datasheet(16/38 Pages) AD

16 / 38 page

LT3757/LT3757A

Rev. G

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 6. The Output Ripple Waveform of a Boost Converter

VOUT

(AC)

tON

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

3757 F06

tOFF

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, and the peak current is equal to:

ID(PEAK) = IL(PEAK) = 1+

⎛

⎝⎜

⎞

⎠⎟ •IL(MAX)

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

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 tem-

perature rating.

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

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 ∆VESR and the charging/discharg-

ing ∆VCOUT. For the purpose of simplicity, we will choose

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

between ∆VESR and ∆VCOUT. 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(PEAK)

capacitors for a given output ripple voltage. The effect of

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

put voltage ripple waveform for a typical boost converter

is illustrated in Figure 6.

LT3757 Datasheet(PDF) 16 Page - Analog Devices

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