Datasheet LT1956, LT1956-5 (Analog Devices) - 9

LT1956/LT1956-5
U U W U APPLICATIO S I FOR ATIO
LT1956 VSW L1 TO FREQUENCY SHIFTING OUTPUT 5V 1.4V Q1 ERROR AMPLIFIER + 1.2V R1 R3 R4 1k 2k FB + – C1 BUFFER R2 Q2 TO SYNC CIRCUIT VC GND 1956 F02
Figure 2. Frequency and Current Limit Foldback
with high input voltage. High frequency pickup will in- VOUT USING crease and the protection accorded by frequency and 10mV/DIV 22µF CERAMIC OUTPUT current foldback will decrease. CAPACITOR
CHOOSING THE INDUCTOR
VOUT USING 100µF, 0.08Ω 10mV/DIV TANTALUM For most applications, the output inductor will fall into the OUTPUT CAPACITOR range of 5µH to 30µH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more V 1 IN = 12V µs/DIV 1956 F03 output current because they reduce peak current seen by VOUT = 5V L = 15µH the LT1956 switch, which has a 1.5A limit. Higher values
Figure 3. LT1956 Output Ripple Voltage Waveforms.
also reduce output ripple voltage.
Ceramic vs Tantalum Output Capacitors
When choosing an inductor you will need to consider Output ripple voltage is determined by ripple current output ripple voltage, maximum load current, peak induc- (ILP-P) through the inductor and the high frequency tor current and fault current in the inductor. In addition, impedance of the output capacitor. At high frequencies, other factors such as core and copper losses, allowable the impedance of the tantalum capacitor is dominated by component height, EMI, saturation and cost should also its effective series resistance (ESR). be considered. The following procedure is suggested as a way of handling these somewhat complicated and con-
Tantalum Output Capacitor
flicting requirements. The typical method for reducing output ripple voltage when using a tantalum output capacitor is to increase the
Output Ripple Voltage
inductor value (to reduce the ripple current in the induc- Figure 3 shows a comparison of output ripple voltage for tor). The following equations will help in choosing the the LT1956 using either a tantalum or ceramic output required inductor value to achieve a desirable output ripple capacitor. It can be seen from Figure 3 that output ripple voltage level. If output ripple voltage is of less importance, voltage can be significantly reduced by using the ceramic the subsequent suggestions in Peak Inductor and Fault output capacitor; the significant decrease in output ripple Current and EMI will additionally help in the selection of voltage is due to the very low ESR of ceramic capacitors. the inductor value. 1956f 9