LTC3729_15 Datasheet, PDF (17/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
OPTIONAL EXTV_CC CONNECTION
5V < V_SEC < 7V
+
C_IN
LTC3729 V_IN
EXTV_CC
TG1
N-CH
SW1
BG1
N-CH
PGND
V_IN
1N4148
6.8V
V_SEC
+
R_SENSE
T1
+
1mF
V_OUT
C_OUT
3729 F05a
Figure 5a. Secondary Output Loop and EXTV_CC Connection
+
C_IN
LTC3729 V_IN
TG1
N-CH
EXTV_CC
SW1
BG1
N-CH
PGND
V_IN
BAT85
+
1µF
0.22µF
BAT85
VN2222LL
R_SENSE
L1
BAT85
V_OUT
+
C_OUT
3729 F05b
Figure 5b. Capacitive Charge Pump for EXTV_CC
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
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
3729fb
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