Datasheet LT3494, LT3494A (Analog Devices) - 9

LT3494/LT3494A
APPLICATIONS INFORMATION Connecting the Load to the CAP Node
If the inductor ripple current is greater than the peak cur- The effi ciency of the converter can be improved by con- rent, then the circuit will only operate in discontinuous necting the load to the CAP pin instead of the V conduction mode. The inductor value should be increased OUT pin. The power loss in the PMOS disconnect circuit is then so that IRIPPLE < IPK. An application circuit can be designed made negligible. By connecting the feedback resistor to to operate only in discontinuous mode, but the output the V current capability will be reduced. OUT pin, no quiescent current will be consumed in the feedback resistor string during shutdown since the PMOS Step 3: Calculate the average input current: transistor will be open (see Figure 3). The disadvantage I of this method is that the CAP node cannot go to ground I = I RIPPLE – amps IN(AVG) PK during shutdown, but will be limited to around a diode 2 drop below VCC. Loads connected to the part should only Step 4: Calculate the nominal output current: sink current.
Never force external power supplies onto the CAP or V
I V • 0.75
OUT pins.
The larger value output capacitor IN(AVG) • IN I = amps (2.2μF to 10μF) should be placed on the node to which OUT NOM ( ) VOUT the load is connected. Step 5: Derate output current: 1 8 C1 ILOAD IOUT = IOUT(NOM) • 0.7 amps SW CAP 3 7 VCC VOUT For low output voltages the output current capability will LT3494 be increased. When using output disconnect (load cur- 5 6 SHDN FB rent taken from V 4 2 OUT), these higher currents will cause CTRL GND the drop in the PMOS switch to be higher resulting in 3494 F03 reduced output current capability than those predicted
Figure 3. Improved Effi ciency
by the preceding equations.
Maximum Output Load Current Inrush Current
The maximum output current of a particular LT3494/ When VCC is stepped from ground to the operating volt- LT3494A circuit is a function of several circuit variables. age while the output capacitor is discharged, a higher The following method can be helpful in predicting the level of inrush current may fl ow through the inductor maximum load current for a given circuit: and integrated Schottky diode into the output capacitor. Step 1: Calculate the peak inductor current: Conditions that increase inrush current include a larger more abrupt voltage step at VIN, a larger output capacitor V tied to the CAP pin and an inductor with a low saturation I I IN = + • • – 400 10 9 amps PK LIMIT current. While the internal diode is designed to handle L such events, the inrush current should not be allowed to where ILIMIT is 0.180A and 0.350A for the LT3494 and exceed 1A. For circuits that use output capacitor values LT3494A respectively. L is the inductance value in Henrys within the recommended range and have input voltages and VIN is the input voltage to the boost circuit. of less than 5V, inrush current remains low, posing no hazard to the device. In cases where there are large steps Step 2: Calculate the inductor ripple current: at VCC (more than 5V) and/or a large capacitor is used V ( + 1 V ) 150 10 9 – • • – at the CAP pin, inrush current should be measured to I OUT IN = amps RIPPLE ensure safe operation. The LT3494A circuits experience L higher levels of current during start-up and steady-state where V operation. An external diode placed from the SW pin to OUT is the desired output voltage. 3494fb 9