Datasheet

LT3973/LT3973-3.3/LT3973-5
16
3973fa
Figure 4. Two Circuits for Generating the Boost Voltage
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
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.
applicaTions inFormaTion
BD
LT3973
(4a) For V
OUT
≥ 2.2V
BOOSTV
IN
V
IN
C3
V
OUT
SW
GND
BD
LT3973
(4b) For V
OUT
< 2.2V; V
IN
< 25V
BOOSTV
IN
V
IN
C3
3973 F04
V
OUT
SW
GND
Minimum Dropout Voltage
When the OUT pin is tied to V
OUT
, the LT3973 regulates
the output such that:
V
IN
– V
OUT
> V
DROPOUT(MIN)
where V
DROPOUT(MIN)
is 530mV. This enforced minimum
dropout voltage keeps the boost capacitor charged regard-
less 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 capacitor. 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 performance. Figure 6 shows
the overall V
IN
to V
OUT
performance during start-up and
dropout conditions.
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 V
OUT
. 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.
LOAD CURRENT (mA)
0 100
2.5
INPUT VOLTAGE (V)
3.5
5.0
200
400
500
3.0
4.5
4.0
300
600
700
TO START/TO RUN
FRONT PAGE APPLICATION
V
OUT
= 3.3V
LOAD CURRENT (mA)
0 100
4.0
INPUT VOLTAGE (V)
5.0
6.5
200
400
500
3973 F05
4.5
6.0
5.5
300
600
700
TO START/TO RUN
FRONT PAGE APPLICATION
V
OUT
= 5V