LTM4602
13
4602fa
Enable
The RUN/SS pin can be driven from logic as shown in
Figure 5. This function allows the LTM4602 to be turned
on or off remotely. The ON signal can also control the
sequence of the output voltage.
Figure 5. Enable Circuit with External Logic
Figure 6. Output Voltage Tracking with the LTC2923 Controller
Output Voltage Tracking
For the applications that require output voltage tracking,
several LTM4602 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 V
OUT
rising and
falling can be implemented with different sets of resistor
values. See the LTC2923 data sheet for more details.
EXTV
CC
Connection
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 LTM4602 can be directly powered by V
IN
. The gate
driver current through LDO is about 18mA. The internal
LDO power dissipation can be calculated as:
P
LDO_LOSS
= 18mA • (V
IN
– 5V)
The LTM4602 also provides an external gate driver volt-
age pin EXTV
CC
. If there is a 5V rail in the system, it is
recommended to connect EXTV
CC
pin to the external 5V
rail. Whenever the EXTV
CC
pin is above 4.7V, the inter-
nal 5V LDO is shut off and an internal 50mA P-channel
switch connects the EXTV
CC
to internal 5V. Internal 5V is
supplied from EXTV
CC
until this pin drops below 4.5V. Do
not apply more than 6V to the EXTV
CC
pin and ensure that
EXTV
CC
< V
IN
. The following list summaries the possible
connections for EXTV
CC
:
1. EXTV
CC
grounded. Internal 5V LDO is always powered
from the internal 5V regulator.
2. EXTV
CC
connected to an external supply. Internal LDO
is shut off. A high effi ciency supply compatible with the
MOSFET gate drive requirements (typically 5V) can im-
prove overall effi ciency. With this connection, it is always
required that the EXTV
CC
voltage can not be higher than
V
IN
pin voltage.
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 effi ciency 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-
RUN/SS
4602 F05
LTM4602
PGND
2N7002
SGND
ON
Q1
V
CC
V
IN
V
IN
R
ONB
V
IN
5V
R
TB1
R
TB2
R
SET
49.9k
1.8V
3.3V
R
TA2
R
TA1
R
ONA
ON
RAMPBUF
TRACK1
TRACK2
FB1
GATE
LTC2923
GND
4602 F06
RAMP
R
SET
66.5k
1.5V
LTM4602
V
IN
V
OUT
LTM4602
DC/DC
V
IN
V
OUT
V
OSET
V
OSET
FB2
SDO
STATUS
APPLICATIONS INFORMATION
