MIC2155_0911 Datasheet, PDF (12/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
Functional Description
The MIC2155 and MIC2156 are two-phase, synchronous
buck controllers operating at a fixed frequency. The two
controllers differ only in switching frequency with the
MIC2155 switching at 500kHz per phase (1MHz at the
input and output) and the MIC2156 switches at 300kHz
per phase.
Some of the advantages of multi-phase operation are:
• Smaller input and output filtering components are
required because of current cancelation and higher
input and output frequency.
• Faster transient response is possible with smaller
output filter component values.
• Load current through each phase is one half the total
output current, which allows for even heat distribution
and smaller components.
• Control circuitry forces better current sharing in the
MOSFETs than paralleling FETs in a single phase
application.
The controller utilizes a voltage-mode control scheme
(VMC). Lossless current sharing is accomplished by
sensing the DC voltage across each inductor winding.
Lossless overcurrent protection is performed by sensing
the voltage across the low-side MOSFET on-resistance
during the off-time.
Other features of the controller are:
• Overvoltage protection
• Soft start
• UVLO
• Enable
• Remote sensing
• Pre-biased output startup
• Multiple input supplies
• Power Good signal
• Frequency synchronization
Startup
A typical startup sequence is shown in Error! Reference
source not found. (also refer to the block diagram). The
enable pins are asserted after V_IN is applied. V_DD is
immediately turned on and an internal FET releases the
soft start pin. The soft start pin controls the error
amplifier voltage. As V_SS ramps up, it reaches a
threshold where the gate drive is enabled and the
MOSFETs start to switch at a very low duty cycle. The
rise of the soft start voltage controls the increase in V_OUT
by gradually allowing the COMP1 pin voltage to rise. A
10mV offset in the current controller keeps the Channel
2 low-side drive off when the output current is low to
prevent current from circulating between the phases.
PGOOD is asserted when V_OUT reaches the PGOOD
threshold.
MIC2155/2156
Figure 1. Startup Sequence
A typical output voltage and inductor current startup is
shown in Figure 2.
Channel 1
Channel 2
Time (4ms/div)
Figure 2. Turn On
Soft Start
The soft-start capacitor controls how fast the output
voltage rises by controlling the COMP pin risetime.
Without soft start a fast or uncontrolled turn-on requires
a higher current from the input source to charge up the
output capacitance.
The soft-start capacitor also controls the delay time
between the enable pin assertion to when V_OUT starts to
rise. Figures 3 and 4 show the soft-start circuitry and
waveform timing.
November 2009
12
M9999-111209-B