LT3971AIMSE-TRPBF Просмотр технического описания (PDF) - Linear Technology

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LT3971AIMSE-TRPBF

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

Linear Technology 

LT3971A/LT3971A-5

APPLICATIONS INFORMATION

To ensure proper Burst Mode operation, the SYNC pin

must be grounded. When synchronized with an external

clock, the LT3971A will pulse skip at light loads. The

quiescent 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 LT3971A output voltage is programmed with a resis-

tor 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.

VOUT Pin

The LT3971A-5 contains an internal 10M feedback resis-

tor divider as well as an internal phase lead capacitor to

feedback the output voltage information to the internal

circuitry. The output will be regulated to 5V when con-

nected directly to the VOUT pin.

An external resistor divider can be added to shift the output

voltage higher than 5V. For example, a USB VBUS supply

programmed to 5.15V allows some voltage drop through

connectors and cables, while keeping the VBUS voltage

within specification at the device. By using the LT3971A-5,

two external 1% resistors program the output to 5.15V

without degrading initial accuracy, which is determined

primarily by the LT3971A-5 output voltage specification.

The external resistor divider must be small compared

to the internal 10M to maintain output voltage accuracy.

The 0.5μA into the VOUT pin is process and temperature

dependent, so will degrade the accuracy of the output

voltage if it is not small compared to the total current in

the external resistor divider. Choose the resistor values

according to:

R1=

VOUT – 5

5 + 0.0005

R2

R1 and R2 refer to the components in Figure 3 in kΩ.

1% resistors should be used to maintain output voltage

accuracy. It should also be noted that the smaller the

resistor values the more the input current will increase

during no load conditions.

LT3971A-5

VOUT

OUTPUT

R1

R2

3971A F03

Figure 3. Resistor Divider Used to Increase the Output Voltage

Above 5V

The VOUT pin has an internal 11V clamp. Output voltage

transients above 11V can be tolerated as long as there is

enough series resistance to limit the current into the 11V

clamp to less than 2mA.

3971af

11

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