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B340A

42V, 2.5A, 2MHz Step-Down Switching Regulator with 2.7µA Quiescent Current

Linear Technology 

LT3975

APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

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

sleep mode the LT3975 consumes 1.7μA, but when it turns

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.

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

performance of the LT3975. The feedback resistors should

preferably be on the order of MΩ and the Schottky catch

900

800

700

600

500

400

300

200

100

0

0

VOUT = 5V

fSW = 800kHz

VOUT = 3.3V

fSW = 600kHz

20 40 60 80 100 120 140 160

LOAD CURRENT (mA)

3975 F01

Figure 1. Switching Frequency in Burst Mode Operation

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.

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.

VSW

5V/DIV

IL

0.5A/DIV

VOUT

10mV/DIV

VIN = 12V

VOUT = 3.3V

ILOAD = 20mA

COUT = 47µF

5µs/DIV

3975 F02

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 a 22µF output capacitor, the output

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.

At higher output loads (above 150mA for the front page

application) the LT3975 will be running at the frequency

programmed by the RT 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 LT3975 will pulse skip at light loads. At very

3975f

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