LTC1929-PG_15 Datasheet, PDF (18/28 Pages) Linear Technology – 2-Phase, High Efficiency, Synchronous Step-Down Switching Regulators

LTC1929/LTC1929-PG
APPLICATIO S I FOR ATIO
after C_SS reaches 4.1V, C_SS begins discharging on the
assumption that the output is in an overcurrent condition.
If the condition lasts for a long enough period as deter-
mined by the size of C_SS, the controller will be shut down
until the RUN/SS pin voltage is recycled. If the overload
occurs during start-up, the time can be approximated by:
t_LO1 ≈ (C_SS • 0.6V)/(1.2µA) = 5 • 10⁵ (C_SS)
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:
t_LO2 ≈ (C_SS • 3V)/(1.2µA) = 2.5 • 10⁶ (C_SS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R_SS, to the RUN/SS pin as
shown in Figure 6. This resistance shortens the soft-start
period and prevents the discharge of the RUN/SS capaci-
tor during a severe overcurrent and/or short-circuit con-
dition. When deriving the 5µA current from V_IN as in the
figure, current latchoff is always defeated. The diode
connecting of this pull-up resistor to INTV_CC, as in
Figure␣ 6, eliminates any extra supply current during shut-
down while eliminating the INTV_CC loading from prevent-
ing controller start-up.
Why should you defeat current latchoff? During the
prototyping stage of a design, there may be a problem with
noise pickup or poor layout causing the protection circuit
to latch off the controller. Defeating this feature allows
troubleshooting of the circuit and PC layout. The internal
short-circuit and foldback current limiting still remains
active, thereby protecting the power supply system from
failure. A decision can be made after the design is com-
plete whether to rely solely on foldback current limiting or
to enable the latchoff feature by removing the pull-up
resistor.
3.3V OR 5V
D1
V_IN
RUN/SS
R_SS*
C_SS
INTV_CC
R_SS*
RUN/SS
D1*
C_SS
The value of the soft-start capacitor C_SS may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C_SS > (C_OUT )(V_OUT)(10^-4)(R_SENSE)
The minimum recommended soft-start capacitor of C_SS =
0.1µF will be sufficient for most applications.
Phase-Locked Loop and Frequency Synchronization
The LTC1929 has a phase-locked loop comprised of an
internal voltage controlled oscillator and phase detector.
This allows the top MOSFET turn-on to be locked to the
rising edge of an external source. The frequency range of
the voltage controlled oscillator is ±50% around the
center frequency f_O. A voltage applied to the PLLFLTR pin
of 1.2V corresponds to a frequency of approximately
220kHz. The nominal operating frequency range of the
LTC1929 is 140kHz to 310kHz.
The phase detector used is an edge sensitive digital type
which provides zero degrees phase shift between the
external and internal oscillators. This type of phase detec-
tor will not lock up on input frequencies close to the
harmonics of the VCO center frequency. The PLL hold-in
range, ∆f_H, is equal to the capture range, ∆f_C:
∆f_H = ∆f_C = ±0.5 f_O (150kHz-300kHz)
The output of the phase detector is a complementary pair
of current sources charging or discharging the external
filter network on the PLLFLTR pin. A simplified block
diagram is shown in Figure 7.
EXTERNAL
OSC
2.4V
PHASE
DETECTOR
PLLIN
DIGITAL
PHASE/
FREQUENCY
50k
DETECTOR
R_LP
10k
C_LP
PLLFLTR
OSC
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
1929 F06
Figure 6. RUN/SS Pin Interfacing
18
1929 F07
Figure 7. Phase-Locked Loop Block Diagram