LT8602 datasheet(22/38 Pages) LINER

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LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

The power dissipated by the diode is given by:

PD =IOUT MAX

(

) • VD

Therefore, a rectifier diode should be chosen with a low

forward voltage drop at peak current for best efficiency

and a reverse breakdown voltage greater than VOUT4(MAX).

If the reverse protection diode (SD2) is needed, the

reverse breakdown voltage must be greater than the

desired reverse polarity protection.

Boost: Output Capacitor

The output capacitor has two essential functions. First, the

output capacitor filters the LT8603’s discontinuous output

current to produce the DC output current. In this role, the

capacitor determines the output ripple, thus low imped-

ance at the switching frequency is important. Second, the

output capacitor stores energy in order to satisfy transient

load conditions and stabilize the LT8603’s control loop.

Typically, the low equivalent series resistance of X5R and

X7R ceramic capacitors provide low output ripple and

good transient response.

For some applications, transient performance can be

improved with higher output capacitance and/or the

addition of a feedforward capacitor placed between the

boost output voltage and the boost feedback pin. Note that

larger output capacitance may be required when lower

switching frequencies are used or when there is signifi-

cant inductance to the load due to long wire or cables.

Increasing the output capacitance will also decrease the

output ripple.

When choosing a capacitor, special attention should be

given to the capacitor’s data sheet to understand the

effective capacitance under the relevant operating condi-

tions of voltage bias and temperature. For good starting

values, refer to the Typical Applications section. For all

applications, careful evaluation of system stability should

be made to ensure adequate design margin.

Boost: Input Capacitor

The input capacitor is in series with the inductor so the

input current waveform is continuous and the di/dt is

limited.

An input capacitor should be chosen to handle the RMS

input capacitor ripple current as given by:

IRMS CIN

(

) = 0.6 • χ • IOUTMAX

Figure 5. Switching Waveforms for a Boost Converter

VBATT

5a. Circuit Diagram

5b. Inductor and Input Currents

COUT

VOUT

IIN

5c. Switch Current

ISW

tON

5d. Diode and Output Currents

5e. Output Voltage Ripple Waveform

tOFF

8603 F05

VOUT

(AC)

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

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