Datasheet

17
LTC1629-6
16296f
Topside MOSFET Driver Supply (C
B
,D
B
) (Refer to
Functional Diagram)
External bootstrap capacitors C
B1
and C
B2
connected to
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C
B
in the
Functional Diagram is charged though diode D
B
from
INTV
CC
when the SW pin is low. When the topside MOSFET
turns on, the driver places the C
B
voltage across the gate-
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to V
IN
and the BOOST pin rises to V
IN
+ V
INTVCC
.
The value of the boost capacitor C
B
needs to be 30 to 100
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of D
B
must be greater
than V
IN(MAX).
The final arbiter when defining the best gate drive ampli-
tude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
Output Voltage
The LTC1629-6 has a true remote voltage sense capablity.
The sensing connections should be returned from the load
back to the differential amplifier’s inputs through a com-
mon, tightly coupled pair of PC traces. The differential
amplifier rejects common mode signals capacitively or
inductively radiated into the feedback PC traces as well
as ground loop disturbances. The differential amplifier
output signal is divided down and compared with the
internal precision 0.6V voltage reference by the error
amplifier.
The output is an NPN emitter follower without any internal
pull-down current. A DC resistive load to ground is re-
quired in order to sink current. The output will swing from
0V to 10V. (V
IN
≥ V
DIFFOUT
␣ +␣ 2V.)
Soft-Start/Run Function
The RUN/SS pin provides three functions: 1) Run/Shut-
down, 2) soft-start and 3) a defeatable short-circuit latchoff
timer. Soft-start reduces the input power sources’ surge
currents by gradually increasing the controller’s current
limit I
TH(MAX)
. The latchoff timer prevents very short,
extreme load transients from tripping the overcurrent
latch. A small pull-up current (>5µA) supplied to the RUN/
SS pin will prevent the overcurrent latch from operating.
The following explanation describes how the functions
operate.
An internal 1.2µA current source charges up the C
SS
capacitor
.
When the voltage on RUN/SS reaches 1.5V, the
controller is permitted to start operating. As the voltage on
RUN/SS increases from 1.5V to 3.0V, the internal current
limit is increased from 25mV/R
SENSE
to 75mV/R
SENSE
.
The output current limit ramps up slowly, taking an
additional 1.4µs/µF to reach full current. The output
current thus ramps up slowly, reducing the starting surge
APPLICATIO S I FOR ATIO
WUU
U
Figure 5a. Secondary Output Loop and EXTV
CC
Connection
Figure 5b. Capacitive Charge Pump for EXTV
CC
1629 F05a
V
IN
TG1
N-CH
1N4148
N-CH
BG1
PGND
LTC1629-6
SW1
EXTV
CC
OPTIONAL EXTV
CC
CONNECTION
5V < V
SEC
< 7V
T1
R
SENSE
V
SEC
6.8V
V
OUT
V
IN
+
C
IN
+
1µF
+
C
OUT
1629 F05b
V
IN
TG1
N-CH
N-CH
BG1
PGND
LTC1629-6
SW1
EXTV
CC
L1
BAT85
BAT85
BAT85 0.22µF
V
OUT
V
IN
+
C
IN
+
+
C
OUT
VN2222LL
R
SENSE