17
LTC3729
sn3729 3729fas
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.
Differential Amplifier/Output Voltage
The LTC3729 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 0.8V voltage reference by the error
amplifier.
The differential amplifier utilizes a set of internal precision
resistors to enable precision instrumentation-type mea-
surement of the output voltage. The output is an NPN
emitter follower without any internal pull-down current. A
DC resistive load to ground is required in order to sink
current. The output will swing from 0V to 10V. (V
IN
≥
V
DIFFOUT
␣ +␣ 2V.) The output voltage is set by an external
resistive divider according to the following formula:
V
V
R
R
OUT
=
+
0 8
1
2
1
.
where R1 and R2 are defined in Figure 2.
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 (>5
µ
A) supplied to the RUN/
SS pin will prevent the overcurrent latch from operating.
The following explanation describes how the functions
operate.
An internal 1.2
µ
A 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 1.4
µ
s/
µ
F to reach full current. The output
current thus ramps up slowly, reducing the starting surge
current required from the input power supply. If RUN/SS
has been pulled all the way to ground there is a delay before
starting of approximately:
t
V
A
C
s
F C
DELAY
SS
SS
=
µ
=
µ
(
)
1 5
1 2
1 25
.
.
.
/
APPLICATIO S I FOR ATIO
W
U
U
U
Figure 5a. Secondary Output Loop and EXTV
CC
Connection
Figure 5b. Capacitive Charge Pump for EXTV
CC
3729 F05a
V
IN
TG1
N-CH
1N4148
N-CH
BG1
PGND
LTC3729
SW1
EXTV
CC
OPTIONAL EXTV
CC
CONNECTION
5V < V
SEC
< 7V
T1
R
SENSE
V
SEC
6.8V
V
OUT
V
IN
+
C
IN
+
1
µ
F
+
C
OUT
3729 F05b
V
IN
TG1
N-CH
N-CH
BG1
PGND
LTC3729
SW1
EXTV
CC
L1
R
SENSE
BAT85
BAT85
BAT85
0.22
µ
F
V
OUT
V
IN
+
C
IN
+
1
µ
F
+
C
OUT
VN2222LL
