LTC1625 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1625' PDF : 24 Pages View PDF
LTC1625
APPLICATIONS INFORMATION
( ) tDELAY
=
1.4V
3µA
CSS
=
0.5s /µF
CSS
When the voltage on RUN/SS reaches 1.4V the LTC1625
begins operating with a clamp on ITH at 0.8V. As the
voltage on RUN/SS increases to approximately 3.1V, the
clamp on ITH is raised until its full 2.4V range is restored.
This takes an additional 0.5s/µF. During this time the load
current will be folded back to approximately 30mV/RDS(ON)
until the output reaches half of its final value.
Diode D1 in Figure 7 reduces the start delay while allowing
CSS to charge up slowly for the soft start function. This
diode and CSS can be deleted if soft start is not needed. The
RUN/SS pin has an internal 6V zener clamp (See Func-
tional Diagram).
3.3V
OR 5V
RUN/SS
D1
CSS
RUN/SS
CSS
1625 F07
Figure 7. RUN/SS Pin Interfacing
FCB Pin Operation
When the FCB pin drops below its 1.19V threshold,
continuous synchronous operation is forced. In this case,
the top and bottom MOSFETs continue to be driven
regardless of the load on the main output. Burst Mode
operation is disabled and current reversal is allowed in the
inductor.
In addition to providing a logic input to force continuous
operation, the FCB pin provides a means to regulate a
flyback winding output. It can force continuous synchro-
nous operation when needed by the flyback winding,
regardless of the primary output load.
The secondary output voltage VSEC is normally set as
shown in Figure 5a by the turns ratio N of the transformer:
VSEC ≅ (N + 1)VOUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
then VSEC will droop. An external resistor divider from
VSEC to the FCB pin sets a minimum voltage VSEC(MIN):
VSEC(MIN)
≅
1.19V 1 +
R4
R3
If VSEC drops below this level, the FCB voltage forces
continuous operation until VSEC is again above its
minimum.
Minimum On-Time Considerations
Minimum on-time tON(MIN) is the smallest amount of time
that the LTC1625 is capable of turning the top MOSFET on
and off again. It is determined by internal timing delays and
the amount of 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
(VIN)(f)
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1625 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple current and ripple voltage will increase.
The minimum on-time for the LTC1625 is generally about
0.5µs. However, as the peak sense voltage (IL(PEAK) •
RDS(ON)) decreases, the minimum on-time gradually
increases up to about 0.7µs. 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 correspondingly larger
current and voltage ripple.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power (×100%). Per-
cent efficiency can be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
15
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