Datasheet LTC3542 (Analog Devices) - 8

LTC3542
APPLICATIONS INFORMATION
A general LTC3542 application circuit is shown in Figure1. the burst clamp. Lower inductor values result in higher External component selection is driven by the load require- ripple current which causes the transition to occur at lower ment and begins with the selection of the inductor L. Once load currents. This causes a dip in effi ciency in the upper the inductor is chosen, CIN and COUT can be selected. range of low current operation. In Burst Mode operation, lower inductance values cause the burst frequency to L VIN V SW V increase. IN 2.7V TO 5.5V OUT CF CIN LTC3542 C
Inductor Core Selection
OUT R2 RUN 3542 F01 VFB Different core materials and shapes change the size/current MODE/SYNC R1 and price/current relationships of an inductor. Toroid or GND shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more
Figure 1. LTC3542 General Schematic
than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use
Inductor Selection
often depends more on the price vs size requirements The inductor value has a direct effect on ripple current ΔIL, and any radiated fi eld/EMI requirements than on what the which decreases with higher inductance and increases with LTC3542 requires to operate. Table 1 shows some typi- higher VIN or VOUT, as shown in following equation: cal surface mount inductors that work well in LTC3542 applications. V ⎛ V ⎞ ΔI OUT OUT L = – 1 L ⎝⎜ V
Input Capacitor (C
⎠⎟ ƒ •
IN) Selection
O IN In continuous mode, the input current of the converter is a where fO is the switching frequency. A reasonable starting square wave with a duty cycle of approximately VOUT/VIN. point for setting ripple current is ΔIL = 0.4 • IOUT(MAX), To prevent large voltage transients, a low equivalent series where IOUT(MAX) is 500mA. The largest ripple current ΔIL resistance (ESR) input capacitor sized for the maximum occurs at the maximum input voltage. To guarantee that RMS current must be used. The maximum RMS capacitor the ripple current stays below a specifi ed maximum, the current is given by: inductor value should be chosen according to the follow- ing equation: V V – V ( ) OUT IN OUT I ≈I ⎛ ⎞ RMS MAX V V OUT OUT VIN L = ⎜ – 1 ⎟ ƒ • I Δ V O L ⎝ IN MA ( X) ⎠ where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple cur- The DC current rating of the inductor should be at least rent, IMAX = ILIM – ΔIL/2. This formula has a maximum at equal to the maximum load current plus half the ripple VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case current to prevent core saturation. Thus, a 600mA rated is commonly used to design because even signifi cant inductor should be enough for most applications (500mA deviations do not offer much relief. Note that capacitor + 100mA). For better effi ciency, chose a low DC-resistance manufacturer’s ripple current ratings are often based on inductor. only 2000 hours life time. This makes it advisable to further The inductor value will also have an effect on Burst Mode derate the capacitor, or choose a capacitor rated at a higher operation. The transition to low current operation begins temperature than required. Several capacitors may also be when the inductor’s peak current falls below a level set by paralleled to meet the size or height requirements of the 3542fa 8