DC1261A Linear Technology, DC1261A Datasheet - Page 11

• Manufacturer Part Number: DC1261A
• Description: BOARD DEMO LTM8022
• Manufacturer: Linear Technology
• Series: µModuler

Specifications of DC1261A

• Design Resources: LTM8022 Spice Model LTM8022 Gerber Files DC1261 Design File DC1261 Schematic
• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Power - Output: -
• Voltage - Output: 3.3V
• Current - Output: 1A
• Voltage - Input: 4.5 ~ 36 V
• Regulator Topology: Buck
• Frequency - Switching: 600kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LTM8022
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Load Sharing
Two or more LTM8022’s may be paralleled to produce higher
currents. To do this, tie the V
all the paralleled LTM8022’s together. Synchronize the
LTM8022s to avoid beat frequencies if required. To ensure
that paralleled modules start up together, the RUN/SS pins
may be tied together, as well. If the RUN/SS pins are not
tied together, make sure that the same valued soft-start
capacitors are used for each module. An example of two
LTM8022 modules confi gured for load sharing is given in
the Typical Applications section.
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8022,
reducing the maximum input current during start-up. The
RUN/SS pin is driven through an external RC fi lter to create
a voltage ramp at this pin. Figure 2 shows the start-up and
shutdown waveforms with the soft-start circuit. By choos-
ing an appropriate RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply at least 20μA when the
RUN/SS pin reaches 2.5V.
Synchronization
The internal oscillator of the LTM8022 can be synchronized
by applying an external 250kHz to 2MHz clock signal to
the SYNC pin. The resistor tied from the R
APPLICATIONS INFORMATION
15k
RUN
0.22μF
Figure 2. To Soft-Start the LTM8022, Add a
Resistor and Capacitor to the RUN/SS Pin
RUN/SS
GND
IN
, V
2ms/DIV
OUT
and SHARE pins of
T
pin to ground
8022 F02
I
0.5A/DIV
V
2V/DIV
V
2V/DIV
L
RUN/SS
OUT
should be chosen such that the LTM8022 would free run
20% lower than the intended synchronization frequency
(see Frequency Selection section).
When the LTM8022 is synchronized to an external clock
source, Burst Mode operation is disabled. The part will
skip power switching cycles as necessary to maintain
regulation. Ensure that the SYNC pin is not left fl oating.
Tie it to GND if not used.
Shorted Input Protection
Care needs to be taken in systems where the output will be
held high when the input to the LTM8022 is absent. This
may occur in battery charging applications, or in battery
backup systems where a battery or some other supply is
diode ORed with the LTM8022’s output. If the V
allowed to fl oat, and the RUN/SS pin is held high (either
by a logic signal or because it is tied to V
LTM8022’s internal circuitry will pull its quiescent current
through its internal power switch. This is fi ne if your system
can tolerate a few milliamps in this state. If you ground
the RUN/SS pin, the internal switch current will drop to
essentially zero. However, if the V
the output is held high, then parasitic diodes inside the
LTM8022 can pull large currents from the output through
the V
the input voltage is present and that protects against a
shorted or reversed input.
Figure 3. The Input Diode Prevents a Shorted Input from
Discharging a Back-Up Battery Tied to the Output. It Also
Protects the Circuit From a Reversed Input. The LTM8022
Runs Only When the Input is Present
V
IN
IN
pin. Figure 3 shows a circuit that will run only when
V
RUN/SS
R
IN
T
GND
LTM8022
SYNC
IN
V
BIAS
ADJ
AUX
OUT
PG
pin is grounded while
8022 F03
LTM8022
IN
), then the
V
IN
OUT
11
pin is
8022fd