ltc1709-7 Linear Technology Corporation, ltc1709-7 Datasheet - Page 19

• Manufacturer Part Number: ltc1709-7
• Description: 2-phase, 5-bit Vid, Current Mode, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to latch off the
controllers when an overcurrent condition is detected. The
RUN/SS capacitor, C
current of both controllers. After the controllers have been
started and been given adequate time to charge up the
output capacitors and provide full load current, the RUN/
SS capacitor is used for a short-circuit timer. If the output
voltage falls to less than 70% of its nominal value after C
reaches 4.1V, C
that the output is in an overcurrent condition. If the
condition lasts for a long enough period as determined by
the size of the C
RUN/SS pin voltage is recycled. If the overload occurs
during start-up, the time can be approximated by:
If the overload occurs after start-up, the voltage on C
continue charging and will provide additional time before
latching off:
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 this pull-up resistor to INTV
eliminates any extra supply current during shutdown
while eliminating the INTV
controller start-up.
t
t
LO1
LO2
3.3V OR 5V
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
(C
(C
SS
SS
D1
Figure 6. RUN/SS Pin Interfacing
• 0.6V)/(1.2 A) = 5 • 10
• 3V)/(1.2 A) = 2.5 • 10
SS
SS
V
IN
, the controller will be shut down until the
begins discharging on the assumption
R
U
SS
SS
*
RUN/SS
, is used initially to limit the inrush
U
C
SS
CC
SS
loading from preventing
, to the RUN/SS pin as
W
D1*
INTV
5
6
CC
(C
(C
CC
R
, as in Figure 6,
SS
SS
SS
*
)
)
RUN/SS
IN
U
as in the
17097 F06
C
SS
SS
will
SS
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.
Phase-Locked Loop and Frequency Synchronization
The LTC1709-7 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
LTC1709-7 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.
C
SS
f
H
= f
> (C
H
, is equal to the capture range, f
C
OUT
= 0.5 f
)(V
O
OUT
. A voltage applied to the PLLFLTR pin
O
)(10
(150kHz-300kHz)
-4
)(R
SENSE
)
LTC1709-7
SS
may need to be
C:
19
SS
=