LT3477IFE データシートの表示（PDF） - Linear Technology

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LT3477IFE

3A, DC/DC Converter with Dual Rail-to-Rail Current Sense

Linear Technology 

LT3477

APPLICATIO S I FOR ATIO

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+ RR21⎞⎠⎟

FBP

LT3477

VREF

FBN

VOUT

R1

R2

3477 F02

Figure 2. Positive Output Voltage Feedback Connections

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⎛⎝⎜ RR34⎞⎠⎟

–VOUT

R3

FBP

LT3477

R4

VREF

FBN

3477 F03

Figure 3. Negative Output Voltage Feedback Connections

Selecting RSENSE/Current Sense Adjustment

Using the following formula to choose the correct current

sense resistor value (for constant current or fail-safe

operation).

RSENSE

=

100mV

ISENSE

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:

ISENSE

=

100mV

RSENSE

•

VIADJ

618mV

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 current-

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

3477fb

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