LT8331 datasheet(23/34 Pages) AD

23 / 34 page

LT8357

Rev. 0

For more information www.analog.com

LT8357

GND

GATEP

GATEN

VIN

VOUT

SENSE

CIN

COUT

RSENSE

CDC

8357 F10

Figure 10. A Simplified SEPIC Converter

In a SEPIC converter, no DC path exists between the input

and output. This is an advantage over the boost converter

for applications requiring the output to be disconnected

from the input source when the circuit is in shutdown.

Compared to the flyback converter, the SEPIC converter

has the advantage that both the power MOSFET and the

output diode voltages are clamped by the capacitors (CIN,

CDC and COUT). Therefore, there is less voltage ringing

across the power MOSFET and the output diode. The

SEPIC converter requires much smaller input capacitors

than those of the flyback converter. This is due to the fact

that, in the SEPIC converter, the inductor L1 is in series

with the input, and the ripple current flowing through the

input capacitor is continuous.

SEPIC Converter: Switch Duty Cycle and Frequency

For a SEPIC converter operating in CCM, the duty cycle

of the main switch can be calculated based on the output

voltage (VOUT), the input voltage (VIN) and the diode for-

ward voltage (VD).

The maximum duty cycle (DMAX) occurs when the con-

verter operates at the minimum input voltage:

DMAX =

VOUT +VD

VIN(MIN)+VOUT +VD

Conversely, the minimum duty cycle (DMIN) occurs when

the converter operates at the maximum input voltage:

APPLICATIONS INFORMATION

DMIN =

VOUT +VD

VIN(MAX)+VOUT +VD

Be sure to check that DMAX and DMIN obey:

DMAX < 1 – TMIN_OFF_TIME(MAX) • f(MAX)

and

DMIN > TMIN_ON_TIME(MAX) • f(MAX)

where Minimum Off-Time TMIN_OFF_TIME, Minimum

On-Time TMIN_ON_TIME and Switching Frequency f are

specified in the Electrical Characteristics table.

SEPIC Converter: Inductor and Sense Resistor Selection

As shown in Figure 10, the SEPIC converter contains two

inductors: L1 and L2. L1 and L2 can be independent, but can

also be wound on the same core, since identical voltages

are applied to L1 and L2 throughout the switching cycle.

For the SEPIC topology, the current through L1 is the

converter input current. Based on the fact that, ideally, the

output power is equal to the input power, the maximum

average inductor currents of L1 and L2 are:

IL1(MAX) =IIN(MAX) =IO(MAX)•

DMAX

1−DMAX

IL2(MAX) =IO(MAX)

In a SEPIC converter, the switch current is equal to IL1 +

IL2 when the power switch is on, therefore, the maximum

average switch current is defined as:

ISW(MAX) =IL1(MAX)+IL2(MAX) =IO(MAX)•

1−DMAX

and the peak switch current is:

ISW(PEAK) =(1+

)•IO(MAX)•

1−DMAX

The constant χ in the preceding equations represents

the percentage peak-to-peak ripple current in the switch,

LT8331 датащи(PDF) 23 Page - Analog Devices

LT8331 датащи(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

датащи скачать

ссылки URL