LTC1929 LINER [Linear Technology], LTC1929 Datasheet - Page 14

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

APPLICATIO S I FOR ATIO
LTC1929
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R
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 5 A current from V
figure, current latchoff is always defeated. The diode
connecting of this pull-up resistor to INTV
Figure 6, eliminates any extra supply current during shut-
down while eliminating the INTV
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.
The value of the soft-start capacitor C
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
The minimum recommended soft-start capacitor of C
0.1 F will be sufficient for most applications.
14
C
3.3V OR 5V
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
SS
> (C
OUT
D1
Figure 6. RUN/SS Pin Interfacing
)(V
V
IN
OUT
R
U
SS
*
RUN/SS
)(10
U
-4
C
SS
)(R
SS
SENSE
CC
, to the RUN/SS pin as
loading from prevent-
W
D1*
)
INTV
SS
CC
may need to be
R
SS
*
IN
RUN/SS
U
CC
as in the
, as in
1929 F06
C
SS
SS
=
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
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
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.
If the external frequency (f
lator frequency f
pulling up the PLLFLTR pin. When the external frequency
is less than f
down the PLLFLTR pin. If the external and internal fre-
quencies are the same but exhibit a phase difference, the
current sources turn on for an amount of time correspond-
ing to the phase difference. Thus the voltage on the
PLLFLTR pin is adjusted until the phase and frequency of
the external and internal oscillators are identical. At this
stable operating point the phase comparator output is
open and the filter capacitor C
LTC1929 PLLIN pin must be driven from a low impedance
source such as a logic gate located close to the pin.
The loop filter components (C
current pulses from the phase detector and provide a
stable input to the voltage controlled oscillator. The filter
components C
acquires lock. Typically R
0.1 F.
f
H
= f
H
, is equal to the capture range, f
C
= 0.5 f
0SC
LP
, current is sunk continuously, pulling
0SC
O
and R
. A voltage applied to the PLLFLTR pin
, current is sourced continuously,
O
(150kHz-300kHz)
LP
LP
PLLIN
determine how fast the loop
=10k and C
LP
) is greater than the oscil-
LP
, R
holds the voltage. The
LP
) smooth out the
LP
C:
is 0.01 F to