LTC1625IGN#TR Linear Technology, LTC1625IGN#TR Datasheet - Page 15

• Manufacturer Part Number: LTC1625IGN#TR
• Description: IC SW REG STEP-DOWN SYNC 16-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1625IGN#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.19 ~ 36 V
• Current - Output: 50mA
• Frequency - Switching: 150kHz
• Voltage - Input: 3.7 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LTC1625IGNTR; Q1127755

APPLICATIONS
When the voltage on RUN/SS reaches 1.4V the LTC1625
begins operating with a clamp on I
voltage on RUN/SS increases to approximately 3.1V, the
clamp on I
This takes an additional 0.5s/ F. During this time the load
current will be folded back to approximately 30mV/R
until the output reaches half of its final value.
Diode D1 in Figure 7 reduces the start delay while allowing
C
diode and C
RUN/SS pin has an internal 6V zener clamp (See Func-
tional Diagram).
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 V
shown in Figure 5a by the turns ratio N of the transformer:
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current,
SS
V
t
DELAY
SEC
to charge up slowly for the soft start function. This
OR 5V
3.3V
(N + 1)V
TH
SS
is raised until its full 2.4V range is restored.
can be deleted if soft start is not needed. The
Figure 7. RUN/SS Pin Interfacing
1 4
3
.
D1
A
V
OUT
U
RUN/SS
C
SS
INFORMATION
C
U
SS
0 5
.
s
/
SEC
W
F C
TH
is normally set as
SS
at 0.8V. As the
RUN/SS
U
1625 F07
C
SS
DS(ON)
then V
V
If V
continuous operation until V
minimum.
Minimum On-Time Considerations
Minimum on-time t
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:
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
0.5 s. However, as the peak sense voltage (I
R
increases up to about 0.7 s. 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:
SEC
DS(ON)
%Efficiency = 100% – (L1 + L2 + L3 + ...)
V
t
SEC
ON MIN
SEC MIN
to the FCB pin sets a minimum voltage V
(
SEC
(
) decreases, the minimum on-time gradually
drops below this level, the FCB voltage forces
)
will droop. An external resistor divider from
)
(
V
V
. 1 19 1
IN
OUT
)( )
ON(MIN)
f
V
is the smallest amount of time
R
R
4
3
SEC
is again above its
LTC1625
SEC(MIN)
L(PEAK) •
15
: