LTC1629C-PG View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC1629C-PG Datasheet PDF : 28 Pages
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LTC1629/LTC1629-PG
APPLICATIO S I FOR ATIO
OPTIONAL EXTVCC CONNECTION
5V < VSEC < 7V
+
VIN
CIN
LTC1629 VIN
TG1
EXTVCC
N-CH
SW1
BG1
1N4148
6.8V
VSEC
+
RSENSE
T1
+
1µF
VOUT
COUT
PGND
N-CH
1629 F05a
Figure 5a. Secondary Output Loop and EXTVCC Connection
+
VIN
CIN
+
CIN
VIN
TG1
LTC1629
EXTVCC
SW1
N-CH
BG1
BAT85 0.22µF
BAT85
VN2222LL
RSENSE
L1
+
BAT85
VOUT
COUT
PGND
N-CH
1629 F05b
Figure 5b. Capacitive Charge Pump for EXTVCC
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to VIN and the BOOST pin rises to VIN + VINTVCC.
The value of the boost capacitor CB needs to be 30 to 100
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of DB must be greater
than VIN(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 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 out-
put signal is divided down and compared with the internal
precision 0.8V voltage reference by the error amplifier.
The differential amplifier can be used in either of two
configurations according to the voltage applied to the
AMPMD pin. The first configuration, with the connections
illustrated in the Functional Diagram, utilizes a set of
internal precision resistors to enable precision instrumen-
tation-type measurement of the output voltage. This con-
figuration is activated when the AMPMD pin is tied to
ground and is the only configuration available for the
LTC1629-PG. When the AMPMD pin is tied to INTVCC, the
resistors are disconnected and the amplifier inputs are
made directly available. The amplifier can then be used as
a general purpose op amp. The amplifier has a 0V to 3V
common mode input range limitation due to the internal
switching of its inputs. 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. (VIN ≥ VDIFFOUT␣ +␣ 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 ITH(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 CSS
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/RSENSE to 75mV/RSENSE.
The output current limit ramps up slowly, taking an
additional 1.4µs/µF to reach full current. The output cur-
17
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