LTM4600(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTM4600 Datasheet PDF : 24 Pages
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LTM4600
APPLICATIO S I FOR ATIO
Enable
The RUN/SS pin can be driven from logic as shown in
Figure 5. This function allows the LTM4600 to be turned
on or off remotely. The ON signal can also control the
sequence of the output voltage.
RUN/SS
ON
LTM4600
PGND SGND
2N7002
4600 F05
Figure 5. Enable Circuit with External Logic
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-
VIN
5V
DC/DC
Q1
3.3V
VIN
RONB
RONA
VCC GATE
ON
RAMP
FB1
LTC2923
VIN
LTM4600
VOSET VOUT
1.8V
49.9k
RTB1
RTA1 RTB2
RTA2
RAMPBUF STATUS
TRACK1
SDO
TRACK2
FB2
GND
VIN
VIN
LTM4600
VOSET VOUT
1.5V
66.5k
4600 F06
Figure 6. Output Voltage Tracking with the LTC2923 Controller
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.
EXTVCC 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 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:
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.
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 pulling 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.
4600fa
13
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