LTC3735 LINER [Linear Technology], LTC3735 Datasheet - Page 20

• Manufacturer Part Number: LTC3735
• Description: 2-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs
• Manufacturer: LINER [Linear Technology]

LTC3735
APPLICATIONS INFORMATION
The masking circuitry also adaptively tracks VID and state
changes. If a new change in VID or state happens before
the 110µs masking timer expires, the timer resets and
starts a fresh count of 110µs. This prevents the system
from rebooting under frequent output voltage transitions.
Refer to Figure 8 for the PGOOD timing diagram.
During start-up, PGOOD is actively pulled low until the RUN/
SS pin voltage reaches its arming voltage, which is 4.2V
typically, only then is the PGOOD pull-low signal released.
When RUN/SS goes low, PGOOD goes low simultaneously.
Fault Conditions: Overcurrent Latchoff
The RUN/SS pin also provides the ability to latch off the
controller when an overcurrent condition is detected. The
RUN/SS capacitor, C
current. After the controller has 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
20
COMPARATOR)
POWER GOOD
INTERNAL PG
(OUTPUT OF
INTERNAL
MASKING
VID BITS
PGOOD
PGOOD
V
OUT
Figure 8. PGOOD Timing Diagram
SS
, is used initially to limit the inrush
110µs
TIME
SS
reaches 4.2V, C
110µs
3735 F08
SS
begins discharging on the assumption 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
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:
If the overload occurs after start-up, the voltage on C
will 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
capacitor during a severe overcurrent and/or short-circuit
condition. When deriving the 5µA current from PV
the figure, current latchoff is always defeated.
Why should you defeat current latchoff? During the pro-
totyping 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 complete
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:
A recommended soft-start capacitor of C
be sufficient for most applications.
t
t
C
LO1
LO2
SS
> (C
≈ (C
≈ (C
OUT
SS
SS
• 0.7V)/(1.5µA) = 4.6 • 10
• 2V)/(1.5µA) = 1.3 • 10
)(V
OUT
)(10
-4
)(R
SS
SENSE
, to the RUN/SS pin as
)
SS
6
(C
5
may need to be
SS
(C
SS
SS
)
= 0.1µF will
)
CC
SS
as in
, the
3735fa
SS