LT3973 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT3973
Linear Technology
'LT3973' PDF : 24 Pages View PDF
LT3973/LT3973-3.3/LT3973-5
APPLICATIONS INFORMATION
than 1.9V above the SW pin for best efficiency. For out-
puts of 2.2V and above, the standard circuit (Figure 4a) is
best. For outputs between 2.2V and 2.5V, use a 1µF boost
capacitor. For output voltages below 2.2V, the boost diode
can be tied to the input (Figure 4b), or to another external
supply greater than 2.2V. However, the circuit in Figure 4a
is more efficient because the BOOST pin current and BD
pin quiescent current come from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
BD
VIN
VIN
BOOST
LT3973
C3
SW
GND
VOUT
5.0
FRONT PAGE APPLICATION
VOUT = 3.3V
4.5
4.0
TO START/TO RUN
3.5
3.0
2.5
0
100 200 300 400 500 600 700
LOAD CURRENT (mA)
6.5
FRONT PAGE APPLICATION
VOUT = 5V
6.0
(4a) For VOUT ≥ 2.2V
5.5
TO START/TO RUN
5.0
BD
VIN
VIN
BOOST
LT3973
C3
SW
VOUT
GND
3973 F04
(4b) For VOUT < 2.2V; VIN < 25V
Figure 4. Two Circuits for Generating the Boost Voltage
The LT3973 monitors the boost capacitor for sufficient
voltage such that the switch is allowed to fully saturate.
During start-up conditions when the boost capacitor may
not be fully charged, the switch will operate with about
1V of drop, and an internal current source will begin to
pull 70mA (typical) from the OUT pin which is typically
connected to VOUT. This current forces the LT3973 to
switch more often and with more inductor current, which
recharges the boost capacitor. When the boost capacitor
is sufficiently charged, the current source turns off, and
the part may enter Burst Mode. See Figure 5 for minimum
input voltage for outputs of 3.3V and 5V.
4.5
4.0
0
100 200 300 400 500 600 700
LOAD CURRENT (mA)
3973 F05
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
Minimum Dropout Voltage
When the OUT pin is tied to VOUT, the LT3973 regulates
the output such that:
VIN – VOUT > VDROPOUT(MIN)
where VDROPOUT(MIN) is 530mV. This enforced minimum
dropout voltage keeps the boost capacitor charged re-
gardless of load during dropout conditions. The LT3973
achieves this by limiting the duty cycle and forcing the
switch to turn off regularly to charge the boost capaci-
tor. Since sufficient voltage across the boost capacitor is
maintained, the switch is allowed to fully saturate and the
internal switch drop stays low for good dropout perfor-
mance. Figure 6 shows the overall VIN to VOUT performance
during start-up and dropout conditions.
16
For more information www.linear.com/LT3973
3973fb
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