Edited by Leland Teschler
Fully integrated POL
power modules have a built-in inductor,
power MOSFET, PWM controller
and supporting circuitry.
It is important to ensure that current
shares evenly when paralleling dc/
dc regulator modules. Conceptually,
part-to-part variations and outputvoltage
regulation errors can make the
output voltages of two
identical POL supplies
differ though the supplies
are programmed
to have the same target
value. For example, a
1.5-V output POL can
have a ±1% error on its
output voltage. When
two POL supplies connect
in parallel, the
output voltage difference
between two
modules can be as high
as 1.5 V 2% = 30 mV. So assuming a
5-mΩ interconnection impedance, the
circulation current can be as high as
30 mV/5 mΩ = 6 A. Without current
sharing, a two-phase, 20-A supply with
two 10-A POLs in parallel might see
one channel providing 16 A and the
other only 4 A. The unbalanced phase
current can cause serious thermal and
reliability issues for the entire system.
A point to note is that the LTM4601
family of μModule POLs has integrated
current-mode controllers to handle
current sharing. The device is a complete
12-A supply housed in a 15 15
2.8-mm LGA package with an IC formfactor
and size. It operates from a 4.5
to 28-V input voltage range. Multiple
LTM4601 modules can easily connect
in parallel for high-current applications.
The accompanying diagram shows how
to connect one LTM4601 (with remote
sensing) and three LTM4601-1 (without
remote sensing) power modules to
devise a 40-A dc/dc step-down supply.
It is possible to parallel multiple
LTM4601 μModule regulators with
balanced phase current. Inside the
μModule, the inductor current is
sensed and compared to the control
signal, Vcomp. The PWM control adjusts
the duty cycle of the dc-dc buck
converter to make the inductor current
follow Vcomp. And the inductor
current increases linearly with
Vcomp. Thus the inductor acts like a
voltage-controlled current source.
Current shares evenly just by connecting
all the current control signal
pins together. In the LTM4601, the COMP pin is both the
output of the op amp that
handles voltage-loop compensation
and the reference
voltage of the inductor
current. The chip uses
transconductance (gm) op
amps, so their outputs (the
COMP pins) can be tied
together.
The concept becomes
clear from a conceptual drawing of
two gm op amps paralleled to generate
a common current reference
voltage, Vcomp. The transconductance
error amplifier’s valid input
range is 10% of the reference voltage.
The Vref is a fixed internal voltage.
For LTM4601, it is 0.6 V. Even
with part-to-part reference voltage
variations of 1%, error amplifiers
are not saturated. In addition, connecting
feedback pins together lets
just one divider resistor set the output
voltage. In the drawing, the upper
resistor is inside the LTM4601. The lower resistor is external and its
value determines the output voltage.
And μModules with a phase-locked
loop (PLL) can each be synchronized
by interleaving clocks to eliminate
beat frequency noise at both
the input and output terminals.
A thermal image of four
LTM4601 modules with 20-V input
and 1.5-V output at 40-A load
current shows well-balanced temperatures
that indicate the modules
share load current well.
— Manjing Xie, Henry Zhang,
Linear Technology Corp.
Make Contact
Linear Technology Corp.,
linear.com
The thermal
image of
four parallel
modules
reveals
an even
distribution of
current (Vin =
20 V, Vo = 1.5 V,
Io = 40 A).