LTM4600
APPLICATIONS INFORMATION
Output Voltage Tracking
1. EXTV grounded. Internal 5V LDO is always powered
CC
from the internal 5V regulator.
For the applications that require output voltage tracking,
several LTM4600 modules can be programmed by the
power supply tracking controller such as the LTC2923.
Figure 6 shows a typical schematic with LTC2923. Coin-
2. EXTV connected to an external supply. Internal LDO
CC
is shut off. A high efficiency supply compatible with the
MOSFET gate drive requirements (typically 5V) can im-
prove overall efficiency. With this connection, it is always
cident, ratiometric and offset tracking for V rising and
O
falling can be implemented with different sets of resistor
required that the EXTV voltage can not be higher than
CC
values. See the LTC2923 data sheet for more details.
V pin voltage.
IN
Discontinuous Operation and FCB Pin
Q1
V
IN
5V
DC/DC
3.3V
The FCB pin determines whether the internal bottom
MOSFET remains on when the inductor current reverses.
There is an internal 4.75k pull-down resistor connecting
this pin to ground. The default light load operation mode
is forced continuous (PWM) current mode. This mode
provides minimum output voltage ripple.
V
V
IN
IN
R
V
GATE
RAMP
FB1
ONB
CC
LTM4600
V
V
1.8V
ON
OSET
OUT
R
ONA
49.9k
LTC2923
STATUS
SDO
RAMPBUF
TRACK1
TRACK2
V
V
In the application where the light load efficiency is im-
portant, tying the FCB pin above 0.6V threshold enables
discontinuous operation where the bottom MOSFET turns
offwheninductorcurrentreverses.Therefore,theconduc-
tionlossisminimizedandlightloadefficiencyisimproved.
The penalty is that the controller may skip cycle and the
output voltage ripple increases at light load.
IN
R
TB1
IN
R
TB2
R
LTM4600
V
TA1
V
FB2
1.5V
OSET
OUT
GND
R
66.5k
TA2
4600 F06
Figure 6. Output Voltage Tracking with the LTC2923 Controller
Paralleling Operation with Load Sharing
EXTV Connection
CC
TwoormoreLTM4600modulescanbeparalleledtoprovide
higher than 10A output current. Figure 7 shows the neces-
saryinterconnectionbetweentwoparalleledmodules.The
OPTI-LOOP® current mode control ensures good current
sharing among modules to balance the thermal stress.
The new feedback equation for two or more LTM4600s
in parallel is:
An internal low dropout regulator produces an internal 5V
supply that powers the control circuitry and FET drivers.
Therefore, if the system does not have a 5V power rail,
the LTM4600 can be directly powered by V . The gate
IN
driver current through LDO is about 18mA. The internal
LDO power dissipation can be calculated as:
P
= 18mA • (V – 5V)
IN
LDO_LOSS
100k
+RSET
N
The LTM4600 also provides an external gate driver volt-
age pin EXTV . If there is a 5V rail in the system, it is
CC
VOUT = 0.6V •
RSET
recommended to connect EXTV pin to the external 5V
CC
rail. Whenever the EXTV pin is above 4.7V, the internal
CC
where N is the number of LTM4600s in parallel.
5V LDO is shut off and an internal 50mA P-channel switch
connects the EXTV to internal 5V. Internal 5V is supplied
CC
OPTI-LOOP is a registered trademark of Linear Technology Corporation.
from EXTV until this pin drops below 4.5V. Do not apply
CC
more than 6V to the EXTV pin and ensure that EXTV
CC
CC
< V . The following list summaries the possible connec-
IN
tions for EXTV :
CC
4600fc
13
Linear [ Linear ]
