Datasheet LT1618 (Analog Devices) - 7

LT1618
U U W U APPLICATIONS INFORMATION Setting Output Voltage
the output of the error amplifier (the VC pin) will be pulled To set the output voltage, select the values of R1 and R2 down and the LT1618 will stop switching. (see Figure 1) according to the following equation. A pulse width modulated (PWM) signal can also be used to adjust the current sense voltage; simply add an RC ⎛ V ⎞ filterto convert the PWM signal into a DC voltage for the R = R OUT 1 2 – 1 ⎜ ⎟ ⎝ 1 263 . ⎠ IADJ pin. If the IADJ pin is not used, it should be tied to ground. Do not leave the pin floating. For current source applications, use the FB pin for over- For applications needing only a simple one-step current voltage protection. Pick R1 and R2 so that the output sense adjustment, the circuit in Figure 2 works well. If a voltage will not go too high if the load is disconnected or large value resistor (≥2MΩ) is placed between the I if the load current drops below the preset value. Typically ADJ pin and ground, the current sense voltage will reduce to about choose R1 and R2 so that the overvoltage value will be 25mV, providing a 50% reduction in current. Do not leave about 20% to 30% higher than the normal output voltage the I (when in constant-current mode). This prevents the volt- ADJ pin open. This method gives a well-regulated current value in both states, and is controlled by a logic age loop from interfering with the current loop in current signal without the need for a variable PWM or DC control source applications. For battery charger applications, pick signal. When the NMOS transistor is on, the current sense the values of R1 and R2 to give the desired end of charge voltage will be 50mV, when it is off, the current sense voltage. voltage will be reduced to 25mV.
Selecting RSENSE/Current Sense Adjustment
LT1618 Use the following formula to choose the correct current sense resistor value (for constant current operation). IADJ RSENSE = 50mV/IMAX FULL 2M CURRENT For designs needing an adjustable current level, the IADJ 1618 F02 pin is provided. With the IADJ pin tied to ground, the nominal current sense voltage is 50mV (appearing be-
Figure 2
tween the ISP and ISN pins). Applying a positive DC voltage to the IADJ pin will decrease the current sense
Considerations When Sensing Input Current
voltage according to the following formula: In addition to regulating the DC output current for current- 1. V 263 – 0 ( 8 . V ) source applications, the constant-current loop of the V IADJ ISENSE = LT1618 can also be used to provide an accurate input 25 current limit. Boost converters cannot provide output For example, if 1V is applied to the IADJ pin, the current short-circuit protection, but the surge turn-on current can sense voltage will be reduced to about 18mV. This be drastically reduced using the LT1618’s current sense adjustability allows the regulated current to be reduced at the input. SEPICs, however, have an output that is DC- without changing the current sense resistor (e.g. to adjust isolated from the input, so an input current limit not only brightness in an LED driver or to reduce the charge current helps soft-start the output but also provides excellent in a battery charger). If the IADJ pin is taken above 1.6V, short-circuit protection. sn1618 1618fas 7