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LT3571

3571fa

Switching Frequency

There are two methods to set the switching frequency of the

LT3571. Both methods require a resistor connected at the

RT pin. Do not leave the RT pin open. Also, do not load this

pin with a capacitor. A resistor must always be connected

for proper operation. One way to set the frequency is simply

connecting an external resistor between the RT pin and

GND. See Table 1 or the Oscillator Frequency vs RT graph

in the Typical Performance Characteristics for resistor

values and corresponding switching frequencies. The other

way is to make the LT3571 synchronize with an external

clock via the SYNC pin. For proper operation, a resistor

should be connected at the RT pin and able to generate

a switching frequency 20% lower than the external clock

when the external clock is absent.

Table 1. Switching Frequency vs RT

Switching Frequency (kHz)

RT (k)

250

56.2

500

26.1

1000

12.1

1500

6.81

2000

4.22

2500

2.67

Inrush Current

The LT3571 has a built-in Schottky diode for the boost

converter. When supply voltage is applied to the VINpin,the

voltage difference between VIN and VOUT generates inrush

current ﬂowing from input through the inductor and the

Schottky diode (D1 in the Block Diagram), to charge the

output capacitor. The selection of inductor and capacitor

value should ensure the peak of the inrush current to below

1A. In addition, the LT3571 turn-on should be delayed until

the inrush current is less than the maximum current limit.

The peak inrush current can be estimated as follows:

IP =

VIN – 0.9

–1

• exp –

2 L

–1

⎛

⎝

⎜

⎞

⎠

⎟

where L is the inductance, and C is the output capacitance.

Table 2 gives inrush peak currents for some component

selections.

Table 2. Inrush Peak Current

VIN (V)

L (μH)

C (μF)

IP (A)

0.81

0.63

Setting Output Voltage

The LT3571 is equipped with both an internal 1V reference

and an auxiliary reference input (the CTRL pin). This

feature allows users to select between using the built-in

reference and supplying an external reference voltage.

The voltage at the CTRL pin can be adjusted while the

chip is operating, to alter the output voltage of LT3571

for purposes such as APD’s bias voltage adjustment. To

use the internal 1V reference, the CTRL pin should be held

higher than 1.2V, which can be done by tying it to VREF.

When the CTRL pin is between 0V and 1V, the LT3571 will

regulate the output such that the FB pin voltage is equal

to the CTRL pin voltage. To set the output voltage, select

the values of R1 and R2 (see Figure 2) according to the

following equation:

R1 = R2

VMONIN

–1

⎛

⎝⎜

⎞

⎠⎟

where V1 = 1V if the internal reference is used, or V1 =

CTRL if CTRL is between 0V and 1V. R2 can be selected to

load the output to maintain a constant switching frequency

when the APD load is very low. Preventing entry into

pulse-skipping mode is an important consideration for

post ﬁltering the regulator output.

Figure 2. Output Voltage Feedback Connection

APPLICATIONS INFORMATION

MONIN

CTRL

3571 F02

LT3571

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