7
DEMO MANUAL DC247
DESIGN-READY SWITCHER
Uninstalled devices Q1, R1, R4 and the V
SEL
pin on the
demo board allow logic selectable output voltages. With
the V
SEL
pin low, the output voltage is set by R7 and R3.
With the V
SEL
pin high, a higher output voltage is set by R7
and the parallel combination of R3 and R4. This creates a
low cost, dynamically programmable mobile processor
power supply.
The demonstration board is shipped with the output
voltage set to 1.6V but may be modified to produce output
voltages as low as 0.8V. Modifications will require changes
to the resistive voltage feedback divider (R4, R7) and, in
some cases, to the I
TH
pin compensation components and
output capacitors (remember their maximum rated volt-
age).
Refer to the LTC1735 data sheet for further information on
the internal operation and functionality of the IC.
OVERCURRENT AND OVERVOLTAGE PROTECTION
The RUN/SS capacitor, C
SS1
,
is initially used to turn on and
limit the inrush current of the controller. After the control-
ler has been started and given adequate time to charge the
output capacitor and provide full load current, C
SS1
is used
as a short-circuit time-out circuit. If the output voltage falls
to less than 70% of its nominal value, C
SS1
begins dis-
charging on the assumption that the output is in an
overcurrent and/or short-circuit condition. If the condition
lasts for a long enough period, as determined by the size
of C
SS1
, the controller will be shut down until the RUN/SS
pin voltage is recycled. This built-in latchoff can be over-
ridden by providing >5µA pullup at a compliance of 4V to
the RUN/SS pin by installing jumper JP2. This current
shortens the soft-start period but also prevents net dis-
charge of the RUN/SS capacitor during an overcurrent
and/or short-circuit condition.
Foldback current limiting is activated when the output
voltage falls below 70% of its nominal level, whether or not
the short-circuit latchoff circuit is enabled.
The output is protected from overvoltage by a “soft-latch.”
When the output voltage exceeds the regulation value by
more than 7.5%, the synchronous MOSFET turns on and
remains on for as long as the overvoltage condition is
present. If the output voltage returns to a safe level, normal
operation resumes. This self-resetting action prevents
“nuisance trips” due to momentary transients and elimi-
nates the need for the Schottky diode that is necessary
with conventional OVP to prevent V
OUT
reversal.
With the overcurrent latchoff enabled, a slow ramp on the
input voltage may cause the circuit to latch off. Simply
recycle the run pin to start. Likewise, if latchoff is disabled
and operation with V
IN
< 5V is anticipated, resistor R6
should be reduced. Refer to the LTC1735 data sheet for
details.
ACTIVE LOADS—BEWARE!
Beware of active loads! They are convenient but problem-
atic. Some active loads do not turn on until the applied
voltage rises above 0.1V to 0.8V. The turn-on may be
delayed as well. A switching regulator with soft-start may
appear to start up, then shut down and eventually reach the
correct output voltage. What happens is as follows: at
switching regulator turn-on, the output voltage is below
the active load’s turn-on requirements. The switching
regulator’s output rises to the correct output voltage level
due to the inherent delay in the active load. The active load
turns on after its internal delay and now pulls down the
switching regulator’s output, because the switcher is in its
soft-start interval. The switching regulator’s output may
come up at some later time when the soft-start interval is
passed.
A switching regulator with foldback current limit will also
have difficulty with the unrealistic I-V characteristic of the
active load. Foldback current limiting will reduce the
output current available as the output voltage drops below
a threshold level (this level is 70% of nominal V
OUT
for the
LTC1735). This reduction in available output current will
result in the active load immediately pulling down the
output because the active load’s current demand remains
constant as the output voltage decreases. Most actual
loads do not behave like the active load I-V characteristics.
Actual loads normally have a V
IN
• C • f dependency, where
C is internal chip capacitance and f is the frequency of
operation. To alleviate the active load problem during
testing, the active load should be initially programmed to
a much lower current value until the switching regulator’s
soft-start interval has passed and then increased to the
OPERATIO
U
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
