LT8602 datasheet(23/38 Pages) LINER

23 / 38 page

LT8603

8603f

For more information www.linear.com/LT8603

APPLICATIONS INFORMATION

Ensure that capacitors present at the input are rated to

withstand any voltage transients that may be applied.

The value of input capacitance is a function of the source

impedance. In general, the higher the source impedance,

the higher the required input capacitance. The Typical

Applications section provides reasonable starting values

for input capacitance but careful evaluation of each appli-

cation must be made to ensure adequate design margin.

SEPIC Configuration

Figure 6 shows the boost controller configured as a sin-

gle-ended primary inductance converter or SEPIC. The

SEPIC configuration offers two primary advantages over

the standard boost configuration. First, it operates like a

buck/boost which means it will regulate to an accurate

output voltage for any input voltage. Second, it offers

short-circuit protection and 0V output in shutdown since

there is no DC path from the input to the output. The dis-

advantage is a more complex circuit requiring additional

components as compared to a standard boost configura-

tion. Since there is no DC path to the output, the LT8603

VIN cannot be connected directly to the SEPIC output. As

a result, diode SD3 is used to ensure start-up, and SD4

is used to ensure operation to low VBATT once started. If

VBATT drops below 4V after the SEPIC is in regulation,

SD4 maintains VIN at the SEPIC output voltage. Just as

with the standard boost configuration, optional diode SD2

provides reverse battery protection.

Figure 7 shows a simplified topology and the current

flow for each of the switch positions once steady state

is reached. Both inductors increase current during the

switch on cycle (Figure 7b). The DC currents of L1 and

L2 are not necessarily equal: IL1(DC) must be equal to

IVBATT(DC) since there is no other DC path for the current

flow from VBATT. By the same argument, IL2(DC) must be

equal to IOUT(DC). Figure 8 shows the current waveforms

of the SEPIC converter.

Although uncoupled inductors can be used in the SEPIC

converter, coupled inductors provide several advantages

and are preferred for most applications. Uncoupled induc-

tors require double the inductance of the coupled induc-

tors and two inductors at twice the inductance usually

cost more than one coupled inductor. Also, uncoupled

inductors form a tank with the coupling capacitor which

can ring at very low frequencies. Uncoupled inductors

can be an advantage, however, in high power or high

duty cycle converters since the current is split between

two cores.

Figure 6. Channel 4 in SEPIC Configuration

8603 F06

SD1

VBATT

LT8603

FB4

GATE4

EN/UVLO

INTVCC4

ISN4

ISP4

BIAS

FSEL4A FSEL4B GND

VIN

SD2*

SD3

*SD2 IS FOR REVERSE INPUT PROTECTION. SHORT IF NOT NEEDED.

INTVCC4

INTVCC

VOUT4 =0.8•

(R2+R3)

SD4

•

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