LT8602 datasheet(26/38 Pages) LINER

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LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

The power dissipated by the diode is:

PD =IOUT(MAX) • VD

If the reverse protection diode, SD2, is needed, the peak

reverse voltage must be greater than the maximum

reverse polarity input expected. The average forward cur-

rent is equal to the input current and the peak forward

current is the same as for the rectifier diode above.

The start-up diodes, SD3 and SD4, have modest current

requirements; the forward current will be less than 50mA

under all input conditions. The peak reverse voltage on

SD3 is the maximum reverse polarity input expected

and the peak reverse voltage on SD4 is the maximum

input voltage.

SEPIC: DC-Coupling Capacitor

The DC-coupling capacitor CC (C5 in Figure 6) sees a nearly

rectangular current waveform as shown in Figure 8c.

During the switch “off” time, the current through CC is

approximately IIN, and approximately –IOUT during the

switch “on” time. This current ripple creates a triangular

ripple voltage on CC:

∆VCC(P−P) =

IOUT(MAX)

CC • fsw

•

VOUT

VBATT + VOUT + VD

The maximum voltage on CC is then:

VCC(MAX) = VBATT(MAX) +

∆VCC(P−P)

which is typically close to VBATT(MAX). The ripple current

through CC is:

IRMS(CC) =IOUT(MAX) •

VOUT + VD

VBATT(MIN)

The capacitance value should be chosen large enough that

∆VCC(P-P) is less than 10% of VBATT(MIN). If ∆VCC(P-P) is

small then the voltage rating is close to VBATT(MAX).

SEPIC: 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

output voltage and the feedback pin. Note that larger out-

put capacitance may be required when lower switching

frequencies are used or when there is significant induc-

tance 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.

SEPIC: 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

(

) = 0.3 •

VBATT MIN

(

)

L • fsw

• DMAX

Ensure that capacitors present at the input are rated to

withstand any voltage transients that may be applied.

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