Datasheet

LTM4627
16
4627fc
For more information www.linear.com/LTM4627
top feedback resistor of the slave regulator in equal slew
rate or coincident tracking, then R
TA
is equal to R
FB
with
V
FB
= V
TRACK
. Therefore R
TB
= 60.4k, and R
TA
= 60.4k in
Figure 4.
In ratiometric tracking, a different slew rate maybe desired
for the slave regulator. R
TB
can be solved for when SR is
slower than MR. Make sure that the slave supply slew
rate is chosen to be fast enough so that the slave output
voltage will reach its final value before the master output.
For example, MR = 1.5V/ms, and SR = 1.2V/ms. Then R
TB
= 75k. Solve for R
TA
to equal 51.1k.
For applications that do not require tracking or sequenc-
ing, simply tie the TRACK/SS pin to INTV
CC
to let RUN
control the turn on/off. When the RUN pin is below its
threshold or the V
IN
undervoltage lockout, then TRACK/
SS is pulled low.
Figure 5. Output Voltage Coincident Tracking
APPLICATIONS INFORMATION
Overcurrent and Overvoltage Protection
The LTM4627 has overcurrent protection (OCP) in a
short circuit. The internal current comparator threshold
folds back during a short to reduce the output current.
An overvoltage condition (OVP) above 10% of the regu
-
lated output voltage will force the top MOSFET off and
the bottom MOSFET on until the condition is cleared.
4627 F05
TIME
SLAVE OUTPUT
MASTER OUTPUT
OUTPUT
VOLTAGE
An input electronic circuit breaker or fuse can be sized to
be tripped or cleared when the bottom MOSFET is turned
on to protect against the overvoltage. Foldback current
limiting is disabled during soft-start or tracking start-up.
Run Enable
The RUN pin is used to enable the power module or
sequence the power module. The threshold is 1.25V, and
the pin has an internal 5.1V Zener to protect the pin. The
RUN pin can be used as an undervoltage lockout (UVLO)
function by connecting a resistor divider from the input
supply to the RUN pin:
V
UVLO
= ((R1+R2)/R2) • 1.25V.
See the Block Diagram for the example of use.
INTV
CC
Regulator
The LTM4627 has an internal low dropout regulator from
V
IN
called INTV
CC
. This regulator output has a 4.7µF
ceramic capacitor internal. This regulator powers the
internal controller and MOSFET drivers. The gate driver
current is
~20mA for 750kHz operation. The regulator loss
can be calculated as:
(V
IN
– 5V) • 20mA = P
LOSS
EXTV
CC
external voltage source ≥ 4.7V can be applied to
this pin to eliminate the internal INTV
CC
LDO power loss and
increase regulator efficiency. A 5V supply can be applied
to run the internal circuitry and power MOSFET driver. If
unused, leave pin floating. EXTV
CC
must be less than V
IN
at all times during power-on and power-off sequences.
Stability Compensation
The LTM4627 has already been internally compensated for
all output voltages. Table 4 is provided for most application
requirements. LTpowerCAD is available for other control
loop optimization.