LTM4600 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4600' PDF : 26 Pages View PDF
LTM4600
Applications Information
Output Voltage Tracking
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-
cident, ratiometric and offset tracking for VO rising and
falling can be implemented with different sets of resistor
values. See the LTC2923 data sheet for more details.
1. EXTVCC grounded. Internal 5V LDO is always powered
from the internal 5V regulator.
2. EXTVCC connected to an external supply. Internal LDO
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
required that the EXTVCC voltage can not be higher than
VIN pin voltage.
VIN
5V
DC/DC
RONB
RONA
RTB1
RTA1 RTB2
RTA2
Q1
3.3V
VCC GATE
ON
RAMP
FB1
LTC2923
RAMPBUF STATUS
TRACK1
SDO
TRACK2
FB2
GND
VIN
VIN
LTM4600
VOSET VOUT
1.8V
49.9k
VIN
VIN
LTM4600
VOSET VOUT
66.5k
4600 F06
1.5V
Figure 6. Output Voltage Tracking with the LTC2923 Controller
Discontinuous Operation and FCB Pin
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.
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
off when inductor current reverses. Therefore, the conduc-
tion loss is minimized and light load efficiency is improved.
The penalty is that the controller may skip cycle and the
output voltage ripple increases at light load.
EXTVCC Connection
Paralleling Operation with Load Sharing
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 VIN. The gate
driver current through LDO is about 18mA. The internal
LDO power dissipation can be calculated as:
PLDO_LOSS = 18mA • (VIN – 5V)
The LTM4600 also provides an external gate driver volt-
age pin EXTVCC. If there is a 5V rail in the system, it is
recommended to connect EXTVCC pin to the external 5V
rail. Whenever the EXTVCC pin is above 4.7V, the internal
5V LDO is shut off and an internal 50mA P-channel switch
connects the EXTVCC to internal 5V. Internal 5V is supplied
from EXTVCC until this pin drops below 4.5V. Do not apply
more than 6V to the EXTVCC pin and ensure that EXTVCC
< VIN. The following list summaries the possible connec-
tions for EXTVCC:
Two or more LTM4600 modules can be paralleled to provide
higher than 10A output current. Figure 7 shows the neces-
sary interconnection between two paralleled modules. 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:
VOUT
=
0.6V
•
100k
N
+
RSET
RSET
where N is the number of LTM4600s in parallel.
4600fd
14
For more information www.linear.com/LTM4600
Share Link: 
