MIC2155_0911 Datasheet, PDF (20/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
MIC2155/2156
E/A 1
V_REF
PWM 1
MIC2155/56
E/A 2
PWM 2
Driver
V_IN
Q1
I_L
Output1 Inductor and
Winding Resistance
R_L1
L1
Driver
Q2
R1
C1
C_O
+ V_S –
10mV V_IN
Q3
I_L
Output2 Inductor and
Winding Resistance
R_L2
L2
Q4
R2
C2
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 V_OUT is less than 0.6V, a
partial discharge of V_OUT 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:
P_IN
=
P_OUT
η
=
50
0.9
= 55W
November 2009
20
M9999-111209-B