Datasheet

LT3480
14
3480fe
For more information www.linear.com/LT3480
APPLICATIONS INFORMATION
Figure 4. Burst Mode Operation
3480 F04
I
L
0.2A/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5µs/DIV
V
IN
= 12V; FRONT PAGE APPLICATION
I
LOAD
= 10mA
Figure 3. Transient Load Response of the LT3480 Front Page
Application as the Load Current Is Stepped from 500mA to
1500mA. V
OUT
= 3.3V
3480 F03
I
L
0.5A/DIV
V
OUT
100mV/DIV
10µs/DIV
V
IN
= 12V; FRONT PAGE APPLICATION
Low-Ripple Burst Mode and Pulse-Skip Mode
The LT3480 is capable of operating in either Low-Ripple
Burst Mode or Pulse-Skip Mode which are selected using
the SYNC pin. See the Synchronization section for details.
To enhance efficiency at light loads, the LT3480 can be
operated in Low-Ripple Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LT3480 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor. Because the LT3480 delivers power to the output
with single, low current pulses, the output ripple is kept
below 15mV for a typical application. In addition, V
IN
and
BD quiescent currents are reduced to typically 30µA and
80µA respectively during the sleep time. As the load cur-
rent decreases towards a no load condition, the percentage
of time that the LT3480 operates in sleep mode increases
and the average input current is greatly reduced resulting
in high efficiency even at very low loads. See Figure 4.
At higher output loads (above 140mA for the front page
application) the LT3480 will be running at the frequency
programmed by the R
T
resistor, and will be operating in
standard PWM mode. The transition between PWM and
Low-Ripple Burst Mode is seamless, and will not disturb
the output voltage.
If low quiescent current is not required the LT3480 can
operate in Pulse-Skip mode. The benefit of this mode is
that the LT3480 will enter full frequency standard PWM
operation at a lower output load current than when in
Burst Mode. The front page application circuit will switch
at full frequency at output loads higher than about 60mA.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.22µF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3V, use a 1µF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1µF boost
capacitor (see Figure 5b). For lower output voltages the
boost diode can be tied to the input (Figure 5c), or to
another supply greater than 2.8V. Tying BD to V
IN
reduces
the maximum input voltage to 30V. The circuit in Figure 5a
is more efficient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT3480 application
is limited by the minimum input voltage (3.6V) and by the
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