MIC2155 datasheet(20/33 Pages) MICREL

20 / 33 page

Micrel, Inc.

MIC2155/2156

May 2009

M9999-052709-A

(408) 944-0800

Output2 Inductor and

Winding Resistance

RL2

VREF

E/A 1

E/A 2

PWM 1

Driver

PWM 2

Driver

VIN

10mV

–

+VS

Output1 Inductor and

Winding Resistance

RL1

MIC2155/56

Figure 18. Current Sharing Diagram

Startup into a Pre-Biased Output

Soft start circuitry in a conventional synchronous buck

regulator forces the regulator to start up by initially

operating at a minimum duty cycle and gradually

increasing the duty cycle until the output voltage reaches

regulation. In a synchronous buck power supply, a

narrow duty cycle means the low-side MOSFET is on for

most of the switching period. If the output voltage is not

0V, the wide on time of the low-side MOSFET may

discharge the output and cause high reverse current to

flow in the inductor.

The MIC2155/6 is designed to turn on into a pre-biased

output without discharging the output. Circuitry in the

controller monitors the input and output voltage and

forces the soft start circuit to initially operate at the

proper duty cycle. This allows the output to turn on in a

controlled fashion without discharging the output. The

minimum output voltage for proper operation of the pre-

bias startup circuitry is 0.6V. If VOUT is less than 0.6V, a

partial discharge of VOUT may occur.

Separate Input Supplies

The MIC2155/6 can operate from two different input

supplies with different voltages. Each of the two

channels can have a different input voltage and still

share current. This allows the supply to draw power from

more than one supply.

The controller will force the output current to be equal.

Since the output voltage and currents of the two

channels are the same, the input power drawn from

each supply will approximately be the same. The input

currents will be inversely proportional to the input

voltages of each supply. For example, if the total output

power is 50W and efficiency is 91%, the total input

power from both supplies is:

OUT

Each supply contributes approximately half the power.

IN =

For VIN1 = 12V and VIN2 = 3.3V

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