Datasheet

LT3971A/LT3971A-5
10
3971af
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3971A operates
in low ripple Burst Mode, which keeps the output capacitor
charged to the desired output voltage while minimizing
the input quiescent current. In Burst Mode operation the
LT3971A delivers single pulses of current to the output
capacitor followed by sleep periods where the output power
is supplied by the output capacitor. When in sleep mode
the LT3971A consumes 1.7A, but when it turns on all the
circuitry to deliver a current pulse, the LT3971A consumes
1.5mA of input current in addition to the switch current.
Therefore, the total quiescent current will be greater than
1.7A when regulating.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3971A is in sleep mode increases, resulting
in much higher light load efficiency. By maximizing the
time between pulses, the converter quiescent current
gets closer to the 1.7A ideal. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider and the reverse current
in the catch diode must be minimized, as these appear
to the output as load currents. Use the largest possible
feedback resistors and a low leakage Schottky catch diode
in applications utilizing the ultralow quiescent current
performance of the LT3971A. The feedback resistors
should preferably be on the order of M and the Schottky
catch diode should have less than 1µA of typical reverse
leakage at room temperature. These two considerations
are reiterated in the FB Resistor Network and Catch Diode
Selection sections.
It is important to note that another way to decrease the
pulse frequency is to increase the magnitude of each
single current pulse. However, this increases the output
voltage ripple because each cycle delivers more power to
the output capacitor. The magnitude of the current pulses
was selected to ensure less than 15mV of output ripple in
a typical application. See Figure 2.
LOAD CURRENT (mA)
SWITCHING FREQUENCY (kHz)
3971A F01
1000
200
400
600
800
0
0 12010080604020
V
IN
= 12V
V
OUT
= 3.3V
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
output capacitance. Therefore, the output voltage ripple
will be inversely proportional to the output capacitance.
In a typical application with a 22F output capacitor, the
output ripple is about 10mV, and with a 47F output ca-
pacitor the output ripple is about 5mV. The output voltage
ripple can continue to be decreased by increasing the
output capacitance.
At higher output loads (above 92mA for the front page
application) the LT3971A 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 operation will exhibit slight frequency
jitter, but will not disturb the output voltage.
Figure 2. Burst Mode Operation
Figure 1. Switching Frequency in Burst Mode Operation
5µs/DIV
3971A F02
V
OUT
20mV/DIV
V
SW
5V/DIV
I
L
500mA/DIV
V
IN
= 12V
V
OUT
= 3.3V
I
LOAD
= 10mA