LTC3734 Datasheet, PDF (19/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
APPLICATIO S I FOR ATIO
enough period as determined by the size of the C_SS, the
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:
t_LO1 ≈ (C_SS • 0.7V)/(1.5μA) = 4.6 • 10⁵ (C_SS)
If the overload occurs after start-up, the voltage on C_SS will
continue charging and will provide additional time before
latching off:
t_LO2 ≈ (C_SS • 2V)/(1.5μA) = 1.3 • 10⁶ (C_SS)
This built-in overcurrent latchoff can be overridden by
providing a pull-up resistor, R_SS, to the RUN/SS pin as
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_CC as in the
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_SS may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C_SS > (C_OUT )(V_OUT)(10^-4)(R_SENSE)
A recommended soft-start capacitor of C_SS = 0.1μF will be
sufficient for most applications.
Minimum On-Time Considerations
Minimum on-time, t_ON(MIN), is the smallest time duration
that the LTC3734 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:
( ) ^{( ) tON MIN}
< ^VOUT
V_IN f
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_OUT(MAX) at V_IN(MAX).
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:
AVP ≅ –35.5 • ^RSENSE • ^R3 ,
(9)
m R_AVP
if
g_m
•
R3
>
10
•
V_OUT
0.6V
3734f
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