LT3482 查看數據表（PDF） - Linear Technology

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LT3482

90V Boost DC/DC Converter with APD Current Monitor

Linear Technology 

LT3482

APPLICATIO S I FOR ATIO

Switching Frequency

The LT3482 can operate at either 650kHz nominal or

1.1MHz nominal; the voltage at the fSET pin selects which

frequency is used. At 1.1MHz, a physically smaller induc-

tor and capacitor can be used in a given application, but

higher frequencies will slightly decrease efficiency and

maximum duty cycle. Generally if efficiency and maximum

duty cycle are crucial, the lower switching frequency should

be selected by connecting fSET to GND. If application size

and cost are more important, connect fSET to VIN to select

the higher switching frequency.

Inrush Current

The LT3482 has built-in Schottky diodes for the boost

and charge pump. When supply voltage is applied to the

VIN pin, the voltage difference between VIN and VOUT1

generates inrush current flowing from input through the

inductor and the Schottky diode (D1 in the Functional

Diagram) to charge the output capacitor. The selection of

inductor and capacitor value should ensure the peak of

the inrush current to be below 1A. In addition, the LT3482

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 – 0.6

L –1

C

•

⎜

exp⎜

⎝⎜⎜

–

2

π

⎟

⎟

L

C

–

1⎠⎟⎟

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

Table 1 gives inrush peak currents for some component

selections.

Table 1. Inrush Peak Current

VIN (V)

5

L (µH)

10

5

22

C (µF)

1

1

IP (A)

0.87

0.68

Setting Output Voltage

The LT3482 is equipped with both an internal 1.235V refer-

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

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 LT3482 for purposes such as

APD’s bias voltage adjustment. To use the internal 1.235V

reference, the CTRL pin should be held higher than 1.5V,

which can be done by tying it to VIN. When the CTRL

pin is between 0V and 1.2V, the LT3482 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 1) according to the following equation:

R1= R2⎛⎝⎜

VOUT2

VREF

– 1⎞⎠⎟

where VREF = 1.235V if the internal reference is used or

VREF = CTRL if CTRL is between 0V and 1.2V. 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.

4

VOUT2

LT3482

R1

13

CTRL

14

FB

R2

3482 F01

Figure 1. Output Voltage Feedback Connection

3482fa

7

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