LTC1929 LINER [Linear Technology], LTC1929 Datasheet - Page 13

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

APPLICATIO S I FOR ATIO
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. 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
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
TH(MAX)
.
When the voltage on RUN/SS reaches 1.5V, the
. The latchoff timer prevents very short,
U
U
SENSE
W
IN
to 75mV/R
V
DIFFOUT
U
CC
SENSE
+ 2V).
, the
SS
.
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
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, after
C
tion that the output is in an overcurrent condition. If the
condition lasts for a long enough period as determined by
the size of C
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
If the overload occurs after start-up the voltage on the
RUN/SS capacitor will continue charging and will provide
additional time before latching off:
SS
T
T
t
t
IRAMP
DELAY
LO1
LO2
reaches 4.1V, C
SS
(C
(C
to ramp up slowly providing the soft-start
SS
SS
SS
1 2
3
1 5
.
V
, the controller will be shut down until the
1 2
.
• 0.6V)/(1.2 A) = 5 • 10
• 3V)/(1.2 A) = 2.5 • 10
.
V
A
1 5
SS
C
.
A
SS
SS
V
begins discharging on the assump-
, is used initially to limit the inrush
C
SS
1 25
.
1 25
s
.
/
s
F C
/
5
6
LTC1929
SS
F C
(C
(C
Q
SS
SS
SS
< 40 A). The
)
)
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