LTC3734EUH Linear Technology, LTC3734EUH Datasheet - Page 19

• Manufacturer Part Number: LTC3734EUH
• Description: IC CTRLR DC/DC 1PH HI EFF 32QFN
• Manufacturer: Linear Technology

Specifications of LTC3734EUH

• Applications: Controller, Intel Mobile CPU
• Voltage - Input: 4 ~ 30 V
• Number Of Outputs: 1
• Voltage - Output: 0.7 ~ 1.71 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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 3. 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
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 LTC3734 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
t
t
C
LO
LO
SS
1
2
> (C
≈ (C
≈ (C
OUT
SS
SS
U
• 2V)/(1.5μA) = 1.3 • 10
• 0.7V)/(1.5μA) = 4.6 • 10
ON(MIN)
)(V
OUT
U
, is the smallest time duration
)(10
SS
-4
, to the RUN/SS pin as
)(R
W
SENSE
SS
SS
may need to be
)
= 0.1μF will be
6
(C
5
CC
U
(C
SS
as in the
SS
)
SS
SS
)
, the
will
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 LTC3734 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 LTC3734 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 equal to or greater
than 15% of I
Active Voltage Positioning
Active voltage positioning can be used to minimize peak-
to-peak output voltage excursion under worst-case tran-
sient loading conditions. The open-loop DC gain of the
control loop is reduced depending upon the maximum
load step specifications. Active voltage positioning can
easily be added to the LTC3734. Figure 6 shows the
equivalent circuit for implementing AVP. The load line
slope is estimated to be:
t
AVP
ON MIN
( )
if g
≅
– . •
m
35 5
<
•
OUT(MAX)
V f
R
V
IN
OUT
3 10
( )
>
R
SENSE
m
at V
•
V
0 6
OUT
.
•
IN(MAX)
R
V
R
AVP
3
.
,
LTC3734
19
3734f
(9)