LT3682 LINER [Linear Technology], LT3682 Datasheet - Page 20

• Manufacturer Part Number: LT3682
• Description: 1A Micropower Step-Down
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3682
Synchronization
To select Low Ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
Synchronizing the LT3682 oscillator to an external
frequency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square wave
amplitude should have valleys that are below 0.3V and
peaks that are above 0.8V (up to 6V).
The LT3682 will not enter Burst Mode at low output loads
while synchronized to an external clock, but instead will
skip pulses to maintain regulation.
The maximum load current that the part can supply is
reduced when a clock signal is applied to SYNC.
The LT3682 may be synchronized over a 300kHz to 2.2MHz
range. The R
switching frequency 20% below the lowest synchronization
input. For example, if the synchronization signal will be
360kHz, the R
reliable and safe operation the LT3682 will only synchronize
when the output voltage is near regulation as indicated
by the PG fl ag. It is therefore necessary to choose a large
enough inductor value to supply the required output current
at the frequency set by the R
Selection section for more information. It is also important
20
500mA/DIV
Figure 8. The LT3682 Reduces its Frequency to
Protect Against Shorted Output with 36V Input
20V/DIV
T
V
SW
0A
resistor should be chosen to set the LT3682
T
I
L
should be chosen for 300kHz. To assure
2μs/DIV
T
resistor. See the Inductor
3682 F08
0V
to note that slope compensation is set by the R
to avoid subharmonics, calculate the minimum inductor
value using the frequency determined by R
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively,
the LT3682 will tolerate a shorted output. When operating
in short-circuit condition, the LT3682 will reduce its
frequency until the valley current is at a typical value of
1.6A (see Figure 8). There is another situation to consider
in systems where the output will be held high when the
input to the LT3682 is absent. This may occur in battery
charging applications or in battery backup systems where
a battery or some other supply is diode OR-ed with the
LT3682’s output. If the V
RUN/SS pin is held high (either by a logic signal or because
it is tied to V
pull its quiescent current through its SW pin. This is fi ne
if your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
essentially zero. However, if the V
the output is held high, then parasitic diodes inside the
LT3682 can pull large currents from the output through
the SW pin and the V
will run only when the input voltage is present and that
protects against a shorted or reversed input.
V
IN
Figure 9. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output.
It Also Protects the Circuit from a Reversed Input.
The LT3682 Runs Only When the Input is Present
MBRS140
D4
IN
), then the LT3682’s internal circuitry will
RUN/SS
V
V
IN
C
GND
LT3682
IN
PGND
BD
pin. Figure 9 shows a circuit that
IN
BOOST
pin is allowed to fl oat and the
FB
SW
DA
IN
pin is grounded while
T
3682 F09
.
BACKUP
V
OUT
T
value:
3682f