LTC1929-PG LINER [Linear Technology], LTC1929-PG Datasheet - Page 17

• Manufacturer Part Number: LTC1929-PG
• Description: 2-Phase, High Efficiency,Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
SW, rises to V
The value of the boost capacitor C
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of D
than V
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 LTC1929 has a true remote voltage sense capability.
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 (LTC1929 only). The first configuration, with
the connections illustrated in the Functional Diagram,
utilizes a set of internal precision resistors to enable
precision instrumentation-type measurement of the out-
put voltage. This configuration is activated when the
AMPMD pin is tied to ground and is the default for the
LTC1929-PG. When the AMPMD pin is tied to INTV
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 (V
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
IN(MAX).
IN
and the BOOST pin rises to V
U
U
B
needs to be 30 to 100
W
IN
B
must be greater
V
DIFFOUT
IN
+ V
U
INTVCC
CC
+ 2V).
, the
.
currents by gradually increasing the controller’s current
limit I
extreme load transients from tripping the overcurrent
latch. A small pull-up current (>5 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 A current source charges up the C
capacitor
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
The output current limit ramps up slowly, taking an
additional 1.4s/ 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:
The time for the output current to ramp up is then:
By pulling both RUN/SS controller pins below 0.8V the
LTC1929 is put into low current shutdown (I
RUN/SS pins can be driven directly from logic as shown in
Figure 6. Diode D1 in Figure 6 reduces the start delay but
allows C
function. The RUN/SS pin has an internal 6V zener clamp
(see Functional Diagram).
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to latch off the
controllers when an overcurrent condition is detected.
The RUN/SS capacitor, C
inrush current of both controllers. After the controllers
have been started and been given adequate time to charge
up the output capacitors and provide full load current, the
RUN/SS capacitor is used for a short-circuit timer. If the
output voltage falls to less than 70% of its nominal value,
t
t
IRAMP
DELAY
TH(MAX)
SS
.
When the voltage on RUN/SS reaches 1.5V, the
to ramp up slowly providing the soft-start
LTC1929/LTC1929-PG
1 2
3
1 5
. The latchoff timer prevents very short,
.
V
1 2
.
.
V
A
1 5
C
.
A
SS
V
C
SS
SS
1 25
, is used initially to limit the
.
1 25
s
.
SENSE
/
s
F C
/
to 75mV/R
SS
F C
Q
SS
< 40 A). The
17
SENSE
SS
.