LT1618 查看數據表（PDF） - Linear Technology

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LT1618

Constant-Current/Constant-Voltage 1.4MHz Step-Up DC/DC Converter

Linear Technology 

LT1618

APPLICATIONS INFORMATION

Setting Output Voltage

To set the output voltage, select the values of R1 and R2

(see Figure 1) according to the following equation.

R1 =

R2

⎛

⎝⎜

VOUT

1.263

–

⎞

1⎠⎟

For current source applications, use the FB pin for over-

voltage protection. Pick R1 and R2 so that the output

voltage will not go too high if the load is disconnected or

if the load current drops below the preset value. Typically

choose R1 and R2 so that the overvoltage value will be

about 20% to 30% higher than the normal output voltage

(when in constant-current mode). This prevents the volt-

age loop from interfering with the current loop in current

source applications. For battery charger applications, pick

the values of R1 and R2 to give the desired end of charge

voltage.

Selecting RSENSE/Current Sense Adjustment

Use the following formula to choose the correct current

sense resistor value (for constant current operation).

RSENSE = 50mV/IMAX

For designs needing an adjustable current level, the IADJ

pin is provided. With the IADJ pin tied to ground, the

nominal current sense voltage is 50mV (appearing be-

tween the ISP and ISN pins). Applying a positive DC

voltage to the IADJ pin will decrease the current sense

voltage according to the following formula:

VISENSE

=

1.263V

– (0.8)VIADJ

25

For example, if 1V is applied to the IADJ pin, the current

sense voltage will be reduced to about 18mV. This

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). If the IADJ pin is taken above 1.6V,

the output of the error amplifier (the VC pin) will be pulled

down and the LT1618 will stop switching.

A pulse width modulated (PWM) signal can also be used

to adjust the current sense voltage; simply add an RC

filterto convert the PWM signal into a DC voltage for the

IADJ pin. If the IADJ pin is not used, it should be tied to

ground. Do not leave the pin floating.

For applications needing only a simple one-step current

sense adjustment, the circuit in Figure 2 works well. If a

large value resistor (≥2MΩ) is placed between the IADJ pin

and ground, the current sense voltage will reduce to about

25mV, providing a 50% reduction in current. Do not leave

the IADJ pin open. This method gives a well-regulated

current value in both states, and is controlled by a logic

signal without the need for a variable PWM or DC control

signal. When the NMOS transistor is on, the current sense

voltage will be 50mV, when it is off, the current sense

voltage will be reduced to 25mV.

LT1618

FULL

CURRENT

IADJ

2M

1618 F02

Figure 2

Considerations When Sensing Input Current

In addition to regulating the DC output current for current-

source applications, the constant-current loop of the

LT1618 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 LT1618’s 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.

sn1618 1618fas

7

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