LT3975
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
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.
To enhance efficiency at light loads, the LT3975 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 LT3975 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
sleepmodetheLT3975consumes1.7μA,butwhenitturns
on all the circuitry to deliver a current pulse, the LT3975
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.
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.
V
SW
5V/DIV
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1) and the percentage
of time the LT3975 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
performanceoftheLT3975. Thefeedbackresistorsshould
preferably be on the order of MΩ and the Schottky catch
I
L
0.5A/DIV
V
OUT
10mV/DIV
3975 F02
V
V
= 12V
5µs/DIV
IN
= 3.3V
= 20mA
= 47µF
OUT
I
LOAD
C
OUT
Figure 2. Burst Mode Operation
While in Burst Mode operation, the burst frequency and
the charge delivered with each pulse will not change with
outputcapacitance.Therefore,theoutputvoltageripplewill
be inversely proportional to the output capacitance. In a
typicalapplicationwitha22µFoutputcapacitor, theoutput
ripple is about 10mV, and with a 47µF output capacitor
the output ripple is about 5mV. The output voltage ripple
can continue to be decreased by increasing the output
capacitance, though care must be taken to minimize the
effects of output capacitor ESR and ESL.
900
800
V
SW
= 5V
OUT
= 800kHz
700
600
500
400
300
200
100
0
f
At higher output loads (above 150mA for the front page
application) the LT3975 will be running at the frequency
programmed by the R 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.
V
SW
= 3.3V
OUT
f
= 600kHz
T
80 100
120 140 160
0
20 40 60
To ensure proper Burst Mode operation, the SYNC pin
must be grounded. When synchronized with an external
clock, the LT3975 will pulse skip at light loads. At very
LOAD CURRENT (mA)
3975 F01
Figure 1. Switching Frequency in Burst Mode Operation
3975f
11
Linear [ Linear ]
