LTM4600
OPERATION
μModule Description
The LTM4600 is a standalone non-isolated synchronous
switching DC/DC power supply. It can deliver up to 10A of
DC output current with only bulk external input and output
capacitors. This module provides a precisely regulated
output voltage programmable via one external resistor from
0.6V
DC
to 5.0V
DC
, not to exceed 80% of the input voltage.
The input voltage range is 4.5V to 20V. A simplified block
diagram is shown in Figure 1 and the typical application
schematic is shown in Figure 18.
The LTM4600 contains an integrated LTC constant on-time
current-mode regulator, ultra-low R
DS(ON)
FETs with fast
switching speed and integrated Schottky diode. The typical
switching frequency is 850kHz at full load. With current
mode control and internal feedback loop compensation,
the LTM4600 module has sufficient stability margins and
good transient performance under a wide range of operat-
ing conditions and with a wide range of output capacitors,
even all ceramic output capacitors (X5R or X7R).
Current mode control provides cycle-by-cycle fast current
limit. In addition, foldback current limiting is provided in
an over-current condition while V
OSET
drops. Also, the
LTM4600 has defeatable short circuit latch off. Internal
overvoltage and undervoltage comparators pull the open-
drain PGOOD output low if the output feedback voltage exits
a ±10% window around the regulation point. Furthermore,
in an overvoltage condition, internal top FET Q1 is turned
off and bottom FET Q2 is turned on and held on until the
overvoltage condition clears.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both Q1 and Q2. Releasing
the pin allows an internal 1.2μA current source to charge
up the softstart capacitor. When this voltage reaches 1.5V,
the controller turns on and begins switching.
At low load current the module works in continuous cur-
rent mode by default to achieve minimum output voltage
ripple. It can be programmed to operate in discontinuous
current mode for improved light load efficiency when the
FCB pin is pulled up above 0.8V and no higher than 6V.
The FCB pin has a 4.75k resistor to ground, so a resistor
to V
IN
can set the voltage on the FCB pin.
When EXTV
CC
pin is grounded or open, an integrated 5V
linear regulator powers the controller and MOSFET gate
drivers. If a minimum 4.7V external bias supply is ap-
plied on the EXTV
CC
pin, the internal regulator is turned
off, and an internal switch connects EXTV
CC
to the gate
driver voltage. This eliminates the linear regulator power
loss with high input voltage, reducing the thermal stress
on the controller. The maximum voltage on EXTV
CC
pin is
6V. The EXTV
CC
voltage should never be higher than the
V
IN
voltage. Also EXTV
CC
must be sequenced after V
IN
.
4600fc
8
LINER [ LINEAR TECHNOLOGY ]
