LTC3735EUHF Linear Technology, LTC3735EUHF Datasheet - Page 21

• Manufacturer Part Number: LTC3735EUHF
• Description: IC CTRLR DC/DC 2PH HI EFF 38-QFN
• Manufacturer: Linear Technology

Specifications of LTC3735EUHF

• Applications: Controller, Intel Mobile CPU
• Number Of Outputs: 1
• Voltage - Output: 0.7 ~ 1.71 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 38-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Input: -

APPLICATIO S I FOR ATIO
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
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
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 5μA current from PV
figure, current latchoff is always defeated.
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
t
t
LO1
LO2
≈ (C
≈ (C
SS
SS
• 0.7V)/(1.5μA) = 4.6 • 10
• 2V)/(1.5μA) = 1.3 • 10
U
U
SS
, to the RUN/SS pin as
W
SS
SS
6
reaches 4.2V, C
(C
5
may need to be
(C
SS
SS
)
CC
)
U
as in the
SS
SS
, the
will
SS
current characteristics. The minimum soft-start capaci-
tance is given by:
A recommended soft-start capacitor of C
sufficient for most applications.
Minimum On-Time Considerations
Minimum on-time, t
that the LTC3735 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty cycle
applications may approach this minimum on-time limit
and care should be taken to ensure that:
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC3735 will begin to skip
cycles resulting in variable frequency operation. The out-
put voltage will continue to be regulated, but the ripple
current and ripple voltage will increase.
The minimum on-time for the LTC3735 is generally less
than 150ns. However, as the peak sense voltage de-
creases, the minimum on-time gradually increases. This is
of particular concern in forced continuous applications
with low ripple current at light loads. If the duty cycle drops
below the minimum on-time limit in this situation, a
significant amount of cycle skipping can occur with corre-
spondingly larger ripple current and ripple voltage.
If an application can operate close to the minimum
on-time limit, an inductor must be chosen that has a low
enough inductance to provide sufficient ripple amplitude
to meet the minimum on-time requirement. As a general
rule, keep the inductor ripple current of each phase equal
to or greater than 15% of I
C
t
ON MIN
SS
(
> (C
)
OUT
<
V f
V
)(V
IN
OUT
( )
OUT
ON(MIN)
)(10
-4
, is the smallest time duration
OUT(MAX)
)(R
SENSE
)
at V
SS
IN(MAX)
LTC3735
= 0.1μF will be
.
21
3735f