LT3971 Просмотр технического описания (PDF) - Linear Technology

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LT3971

38V, 1.2A, 2MHz Step-Down Regulator with 2.8µA Quiescent Current

Linear Technology 

LT3971/LT3971-3.3/LT3971-5

APPLICATIONS INFORMATION

feedback resistors and a low leakage Schottky catch diode

in applications utilizing the ultralow quiescent current

performance of the LT3971. The feedback resistors should

preferably be on the order of MΩ and the Schottky catch

diode should have less than 1µA of typical reverse leak-

age 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

3971 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 LT3971 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.

To ensure proper Burst Mode operation, the SYNC pin

must be grounded. When synchronized with an external

clock, the LT3971 will pulse skip at light loads. The qui-

escent current will significantly increase to 1.5mA in light

load situations when synchronized with an external clock.

Holding the SYNC pin high yields no advantages in terms

of output ripple or minimum load to full frequency, so is

not recommended.

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the resistor

values according to:

R1=

R2





VOUT

1.19V

−

1

Reference designators refer to the Block Diagram. 1%

resistors are recommended to maintain output voltage

accuracy.

The total resistance of the FB resistor divider should be

selected to be as large as possible to enhance low current

performance. The resistor divider generates a small load

on the output, which should be minimized to optimize the

low supply current at light loads.

When using large FB resistors, a 10pF phase lead capacitor

should be connected from VOUT to FB.

The LT3971-3.3 and LT3971-5 contain an internal 10M FB

resistor divider as well as an internal phase lead capacitor.

Setting the Switching Frequency

The LT3971 uses a constant frequency PWM architecture

that can be programmed to switch from 200kHz to 2MHz

by using a resistor tied from the RT pin to ground. A table

showing the necessary RT value for a desired switching

frequency is in Table 1.

3971fd

11

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