lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 11

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The maximum input voltage is determined by the absolute
maximum ratings of the V
frequency operation, the maximum input voltage is de-
termined by the minimum duty cycle DC
DC
restriction on the operating input voltage for fi xed frequency
operation; the circuit will tolerate transient inputs up to the
absolute maximum ratings of the V
Minimum On Time
As the input voltage is increased, the LT3686A is required
to switch for shorter periods of time. Delays associated
with turning off the power switch dictate the minimum on
time of the part. The minimum on time for the LT3686A
is 100ns (Figure 3).
Figure 3. Continuous Mode Operation Near Minimum On Time
MIN
V
IN(MAX)
500mA/DIV
100mV/DIV
can be adjusted with frequency. Note that this is a
10V/DIV
V
V
OUT
SW
AC
=
I
L
V
V
V
L = 6.8μH
C
I
LOAD
OUT
IN
OUT
OUT
DC
= 18V
= 3.3V
= 22μF
= 1.2mA
MIN
+ V
D
– V
IN
D
500ns/DIV
and BOOST pins. For fi xed
+ V
SW
IN
and BOOST pins.
MIN
:
3686A F03
When the required on time decreases below the typical
minimum on time of 100ns, instead of the switch pulse
width becoming narrower to accommodate the lower duty
cycle requirement, the switch pulse width remains fi xed at
100ns. The inductor current ramps up to a value exceed-
ing the load current and the output ripple increases. The
part then remains off until the output voltage dips below
the programmed value before it begins switching again
(Figure 4).
Provided that the load can tolerate the increased output
voltage ripple and that the components have been prop-
erly selected, operation while pulse skipping is safe and
will not damage the part. As the input voltage increases,
the inductor current ramps up quicker, the number of
skipped pulses increases, and the output voltage ripple
increases.
Inductor current may reach current limit when operating
in pulse skip mode with small valued inductors. In this
case, the LT3686A will periodically reduce its frequency
Figure 4. Pulse Skip Occurs When Required On Time Is
Below 100ns
500mA/DIV
100mA/DIV
20V/DIV
V
V
OUT
SW
AC
I
L
V
V
L = 6.8μH
C
I
OUT
IN
OUT
OUT
= 35V
= 300mA
= 3.3V
= 22μF
2μs/DIV
LT3686A
3686A F04
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3686af