LTC1735-1_15 Datasheet, PDF (9/28 Pages) Linear Technology – High Efficiency Synchronous Step-Down Switching Regulator

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OPERATIO (Refer to Functional Diagram)
The main control loop is shut down by pulling Pin 2 (RUN/
SS) low. Releasing RUN/SS allows an internal 1.2µA
current source to charge soft-start capacitor C_SS. When
C_SS reaches 1.5V, the main control loop is enabled with the
I_TH voltage clamped at approximately 30% of its maximum
value. As C_SS continues to charge, I_TH is gradually re-
leased allowing normal operation to resume. If V_OUT has
not reached 70% of its final value when C_SS has charged
to 4.1V, latchoff can be invoked as described in the
Applications Information section.
The internal oscillator can be synchronized to an external
clock applied though a series resistor to the PGOOD pin
and can lock to a frequency between 90% and 130% of its
nominal rate set by capacitor C_OSC.
An overvoltage comparator OV guards against transient
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 limiting for an output shorted to ground
is provided by amplifier A. As V_OSENSE drops below 0.6V,
the buffered I_TH input to the current comparator is gradually
pulled down to a 0.86V clamp. This reduces peak inductor
current to about 1/4 of its maximum value.
Low Current Operation
The LTC1735-1 has three low current modes controlled
by the PGOOD pin. Burst Mode operation is selected when
the PGOOD pin is above 0.8V (typically tied through a
resistor to INTV_CC). During Burst Mode operation, if the
error amplifier drives the I_TH voltage below 0.86V, the
buffered I_TH input to the current comparator will be
clamped at 0.86V. The inductor current peak is then held
at approximately 20mV/R_SENSE (about 1/4 of maximum
output current). If I_TH then drops below 0.5V, the Burst
Mode comparator B will turn off both MOSFETs to maxi-
mize efficiency. The load current will be supplied solely by
the output capacitor until I_TH rises above the 60mV
hysteresis of the comparator and switching is resumed.
Burst Mode operation is disabled by comparator F when
the PGOOD pin is brought below 0.8V. This forces
LTC1735-1
continuous operation and assists in controlling voltage
regulation. If the output voltage is not within 7.5% of its
nominal value the PGOOD open-drain output will be
pulled low and Burst Mode operation will be disabled.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff
The RUN/SS capacitor, C_SS, 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
current, C_SS is used as a short-circuit time-out circuit. If
the output voltage falls to less than 70% of its nominal
output voltage, C_SS begins discharging 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 the C_SS, 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
discharge of C_SS 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.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
of the internal circuitry of the LTC1735-1 is derived from
the INTV_CC pin. When the EXTV_CC pin is left open, an
internal 5.2V low dropout regulator supplies the INTV_CC
power from V_IN. If EXTV_CC is raised above 4.7V, the
internal regulator is turned off and an internal switch
connects EXTV_CC to INTV_CC. This allows a high efficiency
source, such as the primary or a secondary output of the
converter itself, to provide the INTV_CC power. Voltages up
to 7V can be applied to EXTV_CC for additional gate drive
capability.
To provide clean start-up and to protect the MOSFETs,
undervoltage lockout is used to keep both MOSFETs off
until the input voltage is above 3.5V.
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