Datasheet LT1616 (Analog Devices) - 6

LT1616
U U W U APPLICATIO S I FOR ATIO
The LT1616 efficiently converts power from an input volt- (second trace) and the inductor (third trace) increases. age source to a lower output voltage using an inductor for When the switch turns off, the switch current immediately energy storage. The LT1616 uses its internal power switch drops to zero and the inductor current flows through the and an external catch diode (D1 of the application circuit catch diode D1, which clamps the switch node 0.4V below on the first page of this data sheet) to produce a pulse- ground. The voltage across the inductor in this state has width modulated square wave. Inductor L1 and output the opposite sense and is equal to the output voltage plus capacitor C2 filter this square wave to produce a DC output the catch diode drop, so the inductor current begins to voltage. An error amplifier regulates the output by com- decrease. The fourth trace shows the output voltage ripple. paring the output (divided by the feedback resistor string At light loads, the inductor current may reach zero on each R1 and R2) to an internal reference. The LT1616 uses pulse. The diode will turn off, and the switch voltage will current mode control; instead of directly modulating the ring, as shown in Figure 2. This is discontinuous mode op- pulse width, the error amplifier controls the peak current eration, and is normal behavior for the switching regula- in the switch and inductor. Current mode control has sev- tor. The LT1616 will also skip pulses when the load is light. eral advantages, including simplified loop compensation and cycle-by-cycle current limiting. Figure 1 shows several waveforms of the application cir- cuit on the front page of this data sheet. The circuit is V converting a 12V input to 3.3V at 300mA. The first trace is SW 5V/DIV the voltage at the SW pin. When the internal switch is on, the SW pin voltage is near the 12V input. This applies a voltage across inductor L1, and the current in the switch IL1 0.2A/DIV 1616 F02 VIN = 12V 500ns/DIV VOUT = 5V IOUT = 18mA VSW 5V/DIV
Figure 2. Discontinuous Mode Operation
If the output is shorted to ground, the output voltage will ISW collapse and there will be very little voltage to reset the 0.2A/DIV current in the inductor. The LT1616 can sense this condi- tion at its FB pin. In order to control the current, the LT1616 1616 F01a 200ns/DIV reduces its operating frequency, allowing more time for the catch diode to reset the inductor current. The input and output voltages determine the duty cycle of IL1 the switch. The inductor value combined with these volt- 0.2A/DIV ages determines the ripple current in the inductor. Along with the switch current limit, the inductor ripple current VOUT 5mV/DIV determines the maximum load current that the circuit can supply. At minimum, the input and output capacitors are required for stable operation. Specific values are chosen based on allowable ripple and desired transient perfor- 1616 F01b 200ns/DIV mance. The rest of the applications information is mainly
Figure 1. Operating Waveforms of the LT1616
concerned with choosing these and the other components
Converting 12V to 3.3V at 300mA
in an LT1616 application. 6