lt3980 Linear Technology Corporation, lt3980 Datasheet - Page 14

• Manufacturer Part Number: lt3980
• Description: 58v, 2a, 2.4mhz Step-down Switching Regulator With 85?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3980
If low quiescent current is not required the LT3980 can
operate in pulse-skipping mode. The benefi t of this mode
is that the LT3980 will enter full frequency standard PWM
operation at a lower output load current than when in
Burst Mode operation. 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 effi ciency. For
14
4.7μF
V
IN
V
IN
(5a) For V
LT3980
GND
BD
BOOST
SW
OUT
Figure 5. Three Circuits for Generating the Boost Voltage
> 2.8V
C3
4.7μF
V
IN
(5c) For V
V
IN
V
OUT
LT3980
GND
OUT
BD
BOOST
< 2.5V; V
SW
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
the maximum input voltage to 28V. The circuit in Figure 5a
is more effi cient 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.
4.7μF
V
IN(MAX)
IN
C3
= 30V
(5b) For 2.5V < V
V
3980 FO5
IN
V
OUT
LT3980
GND
BD
BOOST
SW
OUT
C3
< 2.8V
D2
V
OUT
IN
reduces
3980f