LT3971IDD-5-PBF 查看數據表（PDF） - Linear Technology

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LT3971IDD-5-PBF

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

Linear Technology 

LT3971/LT3971-3.3/LT3971-5

OPERATION

The LT3971 is a constant frequency, current mode step-

down regulator. An oscillator, with frequency set by RT,

sets an RS flip-flop, turning on the internal power switch.

An amplifier and comparator monitor the current flowing

between the VIN and SW pins, turning the switch off when

this current reaches a level determined by the voltage at

VC (see Block Diagram). An error amplifier measures the

output voltage through an external resistor divider tied to

the FB pin and servos the VC node. If the error amplifier’s

output increases, more current is delivered to the output;

if it decreases, less current is delivered. An active clamp

on the VC node provides current limit. The VC node is

also clamped by the voltage on the SS pin; soft-start is

implemented by generating a voltage ramp at the SS pin

using an external capacitor.

If the EN pin is low, the LT3971 is shut down and draws

700nA from the input. When the EN pin exceeds 1V, the

switching regulator will become active.

The switch driver operates from either VIN or from the

BOOST pin. An external capacitor is used to generate a

voltage at the BOOST pin that is higher than the input

supply. This allows the driver to fully saturate the internal

bipolar NPN power switch for efficient operation.

To further optimize efficiency, the LT3971 automatically

switches to Burst Mode operation in light load situations.

Between bursts, all circuitry associated with controlling

the output switch is shut down, reducing the input supply

current to 1.7μA. In a typical application, 2.8μA will be

consumed from the supply when regulating with no load.

The oscillator reduces the LT3971’s operating frequency

when the voltage at the FB pin is low. This frequency

foldback helps to control the output current during start-

up and overload.

The LT3971 contains a power good comparator which

trips when the FB pin is at 91% of its regulated value. The

PG output is an open-drain transistor that is off when the

output is in regulation, allowing an external resistor to pull

the PG pin high. Power good is valid when the LT3971 is

enabled and VIN is above 4.3V.

APPLICATIONS INFORMATION

Achieving Ultralow Quiescent Current

To enhance efficiency at light loads, the LT3971 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

LT3971 delivers single pulses of current to the output ca-

pacitor followed by sleep periods where the output power

is supplied by the output capacitor. When in sleep mode

the LT3971 consumes 1.7μA, but when it turns on all the

circuitry to deliver a current pulse, the LT3971 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 LT3971 is in sleep mode increases, resulting

in much higher light load efficiency. By maximizing the

time between pulses, the converter quiescent current

1000

VIN = 12V

VOUT = 3.3V

800

600

400

200

0

0 20 40 60 80 100 120

LOAD CURRENT (mA)

3971 F01

Figure 1. Switching Frequency in Burst Mode Operation

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

3971fd

10

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