LTM4622A
9
Rev B
OPERATION
APPLICATIONS INFORMATION
The LTM4622A is a dual output standalone non-isolated
switch mode DC/DC power supply. It can deliver two 2A
DC, 3A peak output current with few external input and
output ceramic capacitors. This module provides dual
precisely regulated output voltage programmable via two
external resistor from 1.5V to 12V over 3.6V to 20V input
voltage range. The typical application schematic is shown
in Figure 27.
The LTM4622A contains an integrated controlled on-time
valley current mode regulator, power MOSFETs, inductors,
and other supporting discrete components. The default
switching frequency is 1MHz. For output voltages above
3.3V, an external resistor is required between FREQ and
GND pins to set the operating frequency to 1.5MHz to
2MHz to optimize inductor current ripple. For switching
noise-sensitive applications, the switching frequency can
be adjusted by external resistors and the μModule regulator
can be externally synchronized to a clock within ±30% of
the set frequency. See the Applications Information section.
With current mode control and internal feedback loop
compensation, the LTM4622A module has sufficient
stability margins and good transient performance with
a wide range of output capacitors, even with all ceramic
output capacitors.
Current mode control provides cycle-by-cycle fast cur-
rent limiting. An internal overvoltage and undervoltage
comparators pull the open-drain PGOOD output low if the
output feedback voltage exits a ±8% window around the
regulation point. Furthermore, an input overvoltage protec-
tion been utilized by shutting down both power MOSFETs
when V
IN
rises above 22.5V to protect internal devices.
Multiphase operation can be easily employed by connecting
SYNC pin to an external oscillator. Up to 6 phases can be
paralleled to run simultaneously a good current sharing
guaranteed by current mode control loop.
Pulling the RUN pin below 1V forces the controller into
its shutdown state, turning off both power MOSFETs and
most of the internal control circuitry. At light load currents,
Burst Mode operation can be enabled to achieve higher
efficiency compared to continuous mode (CCM) by set-
ting MODE pin to INTV
CC
. The TRACK/SS pin is used for
power supply tracking and soft-start programming. See
the Applications Information section.
The typical LTM4622A application circuit is shown in
Figure 27. External component selection is primarily
determined by the input voltage, the output voltage and
the maximum load current. Refer to Table 7 for specific
external capacitor requirements for a particular application.
V
IN
to V
OUT
Step-Down Ratios
There are restrictions in the maximum V
IN
and V
OUT
step
down ratio that can be achieved for a given input voltage
due to the minimum off-time and minimum on-time limits
of the regulator. The minimum off-time limit imposes a
maximum duty cycle which can be calculated as:
DC
(MAX)
= 1 – t
OFF(MIN)
• f
SW
where t
OFF(MIN)
is the minimum off-time, 45ns typical for
LTM4622A, and f
SW
is the switching frequency. Conversely
the minimum on-time limit imposes a minimum duty cycle
of the converter which can be calculated as:
DC
(MIN)
= t
ON(MIN)
• f
SW
where t
ON(MIN)
is the minimum on-time, 20ns typical for
LTM4622A. In the rare cases where the minimum duty
cycle is surpassed, the output voltage will still remain
in regulation, but the switching frequency will decrease
from its programmed value. Note that additional thermal
derating may be applied. See the Thermal Considerations
and Output Current Derating section in this data sheet.
