LTC1735
U
(Refer to Functional Diagram)
OPERATIO
The main control loop is shut down by pulling Pin 2
(RUN/SS)low.ReleasingRUN/SSallowsaninternal1.2µA
current source to charge soft-start capacitor CSS. When
CSS reaches1.5V,themaincontrolloopisenabledwiththe
When the FCB pin is driven by an external oscillator, a low
noise cycle-skipping mode is invoked and the internal
oscillator is synchronized to the external clock by com-
parator C. In this mode the 25% minimum inductor
current clamp is removed, providing constant frequency
discontinuous operation over the widest possible output
current range. This constant frequency operation is not
quite as efficient as Burst Mode operation, but provides a
lower noise, constant frequency spectrum.
I
TH voltageclampedatapproximately30%ofitsmaximum
value. As CSS continues to charge, ITH is gradually re-
leased allowing normal operation to resume. If VOUT has
not reached 70% of its final value when CSS has charged
to 4.1V, latchoff can be invoked as described in the
Applications Information section.
Tying the FCB pin to ground enables forced continuous
operation. This is the least efficient mode, but is desirable
in certain applications. The output can source or sink
current in this mode. When sinking current while in forced
continuous operation, current will be forced back into the
mainpowersupplypotentiallyboostingtheinputsupplyto
dangerous voltage levels—BEWARE.
The internal oscillator can be synchronized to an external
clock applied to the FCB pin and can lock to a frequency
between 90% and 130% of its nominal rate set by capaci-
tor COSC
.
Anovervoltagecomparator, OV, guardsagainsttransient
overshoots (>7.5%) as well as other more serious
conditions that may overvoltage the output. In this case,
the top MOSFET is turned off and the bottom MOSFET is
turned on until the overvoltage condition is cleared.
Foldback Current, Short-Circuit Detection and
Short-Circuit Latchoff
The RUN/SS capacitor, CSS, is used initially to limit the
inrush current of the switching regulator. After the con-
troller has been started and been given adequate time to
charge up the output capacitors and provide full load cur-
rent, CSS is used as a short-circuit time-out circuit. If the
output voltage falls to less than 70% of its nominal output
voltage, CSS begins discharging on the assumption that
the output is in an overcurrent and/or short-circuit condi-
tion. If the condition lasts for a long enough period as
determined by the size of CSS, the controller will be shut
down until the RUN/SS pin voltage is recycled. This built-
in latchoff can be overridden by providing a current >5µA
at a compliance of 5V to the RUN/SS pin. This current
shortens the soft-start period but also prevents net dis-
charge of CSS during an overcurrent and/or short-circuit
condition. Foldback current limiting is activated when the
outputvoltagefallsbelow70%ofitsnominallevelwhether
or not the short-circuit latchoff circuit is enabled.
Foldback current limiting for an output shorted to ground
is provided by amplifier A. As VOSENSE drops below 0.6V,
the buffered ITH input to the current comparator is gradu-
ally pulled down to a 0.86V clamp. This reduces peak
inductor current to about 1/4 of its maximum value.
Low Current Operation
The LTC1735 has three low current modes controlled by
the FCB pin. Burst Mode operation is selected when the
FCB pin is above 0.8V (typically tied to INTVCC). In Burst
Modeoperation,iftheerroramplifierdrivestheITH voltage
below 0.86V, the buffered ITH input to the current com-
parator will be clamped at 0.86V. The inductor current
peak is then held at approximately 20mV/RSENSE (about
1/4 of maximum output current). If ITH then drops below
0.5V, the Burst Mode comparator B will turn off both
MOSFETs to maximize efficiency. The load current will be
supplied solely by the output capacitor until ITH rises
above the 60mV hysteresis of the comparator and switch-
ing is resumed. Burst Mode operation is disabled by
comparator F when the FCB pin is brought below 0.8V.
This forces continuous operation and can assist second-
ary winding regulation.
1735fc
10
Linear [ Linear ]
