Datasheet

LTC3729
17
3729fb
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 amplitude
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 common,
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 preci‑
sion resistors to enable precision instrumentation‑type
measurement 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
APPLICATIONS INFORMATION
current. The output voltage is set by an external resistive
divider according to the following formula:
V
OUT
= 0.8V 1+
R1
R2






where R1 and R2 are defined in the Functional Diagram.
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, ex‑
treme 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 ad‑
ditional 1.4µs/µF 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
Figure 5a. Secondary Output Loop and EXTV
CC
Connection
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
+
1mF
+
C
OUT
Figure 5b. Capacitive Charge Pump for EXTV
CC
3729 F05b
V
IN
TG1
N-CH
N-CH
BG1
PGND
LTC3729
SW1
EXTV
CC
L1
R
SENSE
BAT85
BAT85
BAT85 0.22µF
V
OUT
V
IN
+
C
IN
+
1µF
+
C
OUT
VN2222LL