Datasheet

LT3995
11
3995f
For more information www.linear.com/LT3995
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT3995 operates
in low ripple Burst Mode operation, which keeps the out-
put capacitor charged to the desired output voltage while
minimizing the input quiescent current. In Burst Mode
operation the LT3995 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 LT3995 consumes 1.7μA, but when it turns
on all the circuitry to deliver a current pulse, the LT3995
consumes several mA of input current in addition to the
switch current. Therefore, the total quiescent current will
be greater than 1.7μA when regulating.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3995 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 1.7μA 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 LT3995. The feedback resistors should
preferably be on the order of MΩ and the Schottky catch
diode should have less than a few µA of typical reverse
leakage at room temperature. These two considerations
are reiterated in the FB Resistor Network and Catch Diode
Selection sections.
Figure 1. Switching Frequency in Burst Mode Operation
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 30mV of output ripple
with one 47µF ceramic output capacitor in a typical ap-
plication. See Figure 2.
Figure 2. Burst Mode Operation
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 two 47µF output capacitors,
the output ripple is about 15mV, and with four 47µF output
capacitors the output ripple is about 7.5mV. The output
voltage ripple can continue to be decreased by increas-
ing the output capacitance, though care must be taken
to minimize the effects of output capacitor ESR and ESL.
At higher output loads (above 90mA for the front page
application) the LT3995 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 is seamless, and will not
disturb the output voltage.
To ensure proper Burst Mode operation, the SYNC pin must
be grounded. When synchronized with an external clock,
the LT3995 will pulse skip at light loads. At very light loads,
the part will go to sleep between groups of pulses, so the
quiescent current of the part will still be low, but not as
low as in Burst Mode operation. The quiescent current in
a typical application when synchronized with an external
LOAD CURRENT (mA)
0
50
SWITCHING FREQUENCY (kHz)
100
200
300
400
600
20
40 60 80
3995 F01
100 120
500
V
OUT
= 5V
f
SW
= 500kHz
L = 10µH
V
OUT
= 3.3V
f
SW
= 300kHz
L = 8.2µH
V
IN
= 12V
V
SW
20V/DIV
V
OUT
50mV/DIV
I
L
1A/DIV
5µs/DIVV
IN
= 48V
V
OUT
= 3.3V
I
LOAD
= 70mA
C
OUT
= 47µF
3995 F02