Datasheet

LT3971A/LT3971A-5
18
3971af
APPLICATIONS INFORMATION
Be aware that when the input voltage is below 4.3V, the
input current may rise to several hundred A. And the part
may be able to switch at cold or for V
IN(EN
) thresholds less
than 7V. Figure 7 shows the magnitude of the increased
input current in a typical application with different pro-
grammed V
IN(EN)
.
When operating in Burst Mode for light load currents, the
current through the V
IN(EN)
resistor network can easily be
greater than the supply current consumed by the LT3971A.
Therefore, the V
IN(EN)
resistors should be large to minimize
their effect on efficiency at low loads.
Soft-Start
The SS pin can be used to soft-start the LT3971A by
throttling the maximum input current during start-up. An
internal 1A current source charges an external capaci-
tor generating a voltage ramp on the SS pin. The SS pin
clamps the internal V
C
node, which slowly ramps up the
current limit. Maximum current limit is reached when
the SS pin is about 1.5V or higher. By selecting a large
enough capacitor, the output can reach regulation without
overshoot. For applications with input voltages above 25V,
a 100k resistor in series with the soft-start capacitor is
recommended. Figure 8 shows start-up waveforms for a
typical application with a 10nF capacitor on SS for a 3.3Ω
load when the EN pin is pulsed high for 13ms.
The external SS capacitor is only actively discharged when
EN is low. With EN low, the external SS cap is discharged
through approximately 150Ω. The EN pin needs to be low
long enough for the external cap to completely discharge
through the 150Ω pull-down prior to start-up.
Figure 7. Input Current vs Input Voltage
for a Programmed V
IN(EN)
of 6V and 12V
3971A F07
INPUT VOLTAGE (V)
12V V
IN(EN)
Input Current
6V V
IN(EN)
Input Current
01234
INPUT CURRENT (µA)
300
400
12
200
100
0
65 7 8 9 10 11
500
300
400
200
100
0
500
INPUT VOLTAGE (V)
01234
INPUT CURRENT (µA)
65
V
IN(EN)
= 6V
R3 = 5M
R4 = 1M
V
IN(EN)
= 12V
R3 = 11M
R4 = 1M
Synchronization
To select low ripple Burst Mode operation, tie the SYNC
pin below 0.6V (this can be ground or a logic low output).
Synchronizing the LT3971A oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.6V
and peaks above 1.0V (up to 6V).
2ms/DIV
3971A F08
V
SS
1V/DIV
V
OUT
2V/DIV
I
L
0.5A/DIV
Figure 8. Soft-Start Waveforms for Front-Page Application
with 10nF Capacitor on SS. EN is Pulsed High for About
13ms with a 3.3Ω Load Resistor