LT3971AIMSE-TRPBF View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3971AIMSE-TRPBF

38V, 1.3A, 2MHz Step-Down Regulator with 2.2µA Quiescent Current

Linear Technology 

LT3971A/LT3971A-5

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 1.7μA, 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

1.7μA when regulating.

As the output load decreases, the frequency of single cur-

rent pulses decreases (see Figure 1) and the percentage

of time the 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 1.7μA 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 LT3971A. The feedback resistors

should preferably be on the order of MΩ and the Schottky

catch diode should have less than 1μA of typical reverse

1000

VIN = 12V

VOUT = 3.3V

800

600

400

200

0

0 20 40 60 80 100 120

LOAD CURRENT (mA)

3971A F01

Figure 1. Switching Frequency in Burst Mode Operation

10

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

500mA/DIV

VOUT

20mV/DIV

VIN = 12V

VOUT = 3.3V

ILOAD = 10mA

5μs/DIV

3971A 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 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 RT 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.

3971af

Share Link: