LT3477 datasheet(9/20 Pages) LINER

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LT3477

3477fc

APPLICATIONS INFORMATION

Setting Positive Output Voltages

To set a positive output voltage, select the values of R1 and

R2 (see Figure 2) according to the following equation:

VOUT =1.235V 1+

For designs needing an adjustable current level, the

IADJ1 and IADJ2 pins are provided for the ﬁrst and the

second current sense ampliﬁers, respectively. With the

IADJ1 and IADJ2 pins tied higher than 650mV, the nominal

current sense voltage is 100mV (appearing between the

ISP1 and ISN2 or ISP2 and ISN2 pins). Applying a positive

DC voltage less than 600mV to the IADJ1 and IADJ2 pins

will decrease the current sense voltage according to the

following formula:

SENSE

IADJ

100

618

•

For example, if 309mV is applied to the IADJ1 pin and

RSENSE is 0.5Ω, the current sense will be reduced from

200mA to 100mA. The adjustability allows the regulated

current to be reduced without changing the current sense

resistor (e.g., to adjust brightness in an LED driver or to

reduce the charge current in a battery charger).

Considerations When Sensing Input Current

In addition to regulating the DC output current for cur-

rent-source applications, the constant-current loop of

the LT3477 can also be used to provide an accurate input

current limit. Boost converters cannot provide output

short-circuit protection, but the surge turn-on current can

be drastically reduced using the LT3477 current sense at

the input. SEPICs, however, have an output that is DC-

isolated from the input, so an input current limit not only

helps soft-start the output but also provides excellent

short-circuit protection.

When sensing input current, the sense resistor should be

placed in front of the inductor (between the decoupling

capacitor and the inductor). This will regulate the average

inductor current and maintain a consistent inductor ripple

current, which will, in turn, maintain a well regulated input

current. Do not place the sense resistor between the input

source and the input decoupling capacitor, as this may allow

the inductor ripple current to vary widely (even though the

average input current and the average inductor current will

still be regulated). Since the inductor current is a triangular

waveform (not a DC waveform like the output current)

Figure 2. Positive Output Voltage Feedback Connections

3477 F03

FBP

VREF

–VOUT

LT3477

FBN

Figure 3. Negative Output Voltage Feedback Connections

3477 F02

FBN

VREF

VOUT

LT3477

FBP

Setting Negative Output Voltages

To set a negative output voltage, select the values of R3 and

R4 (see Figure 3) according to the following equation:

VOUT = –1.235V

Selecting RSENSE/Current Sense Adjustment

Using the following formula to choose the correct current

sense resistor value (for constant current or fail-safe

operation).

SENSE

100

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