Datasheet LTC3642 (Analog Devices) - 10

LTC3642
APPLICATIONS INFORMATION
The basic LTC3642 application circuit is shown on the front maximum average output current for this architecture page of this data sheet. External component selection is is limited to half of the peak current. Therefore, be sure determined by the maximum load current requirement and to select a value that sets the peak current with enough begins with the selection of the peak current programming margin to provide adequate load current under all foresee- resistor, RISET. The inductor value L can then be determined, able operating conditions. followed by capacitors CIN and COUT.
Inductor Selection Peak Current Resistor Selection
The inductor, input voltage, output voltage and peak current The peak current comparator has a maximum current determine the switching frequency of the LTC3642. For limit of 115mA nominally, which results in a maximum a given input voltage, output voltage and peak current, average current of 55mA. For applications that demand the inductor value sets the switching frequency when the less current, the peak current threshold can be reduced output is in regulation. A good first choice for the inductor to as little as 25mA. This lower peak current allows the value can be determined by the following equation: use of lower value, smaller components (input capacitor, output capacitor and inductor), resulting in lower input  V   V  L = OUT   • 1– OUT  supply ripple and a smaller overall DC/DC converter.  f • I  V PEAK  IN  The threshold can be easily programmed with an ap- The variation in switching frequency with input voltage propriately chosen resistor (RISET) between the ISET pin and inductance is shown in the following two figures for and ground. The value of resistor for a particular peak typical values of V current can be computed by using Figure 1 or the follow- OUT. For lower values of IPEAK, multiply the frequency in Figure 2 and Figure 3 by 115mA/I ing equation: PEAK. An additional constraint on the inductor value is the RISET = IPEAK • 9.09 • 106 LTC3642’s 100ns minimum on-time of the high side switch. where 25mA < IPEAK < 115mA. Therefore, in order to keep the current in the inductor well The peak current is internally limited to be within the controlled, the inductor value must be chosen so that it is range of 25mA to 115mA. Shorting the I larger than LMIN, which can be computed as follows: SET pin to ground programs the current limit to 25mA, and leaving it floating VIN(MAX) • tON(MIN) sets the current limit to the maximum value of 115mA. LMIN = I When selecting this resistor value, be aware that the PEAK(MAX) where VIN(MAX) is the maximum input supply voltage for 1100 the application, tON(MIN) is 100ns, and IPEAK(MAX) is the 1000 maximum allowed peak inductor current. Although the 900 800 above equation provides the minimum inductor value, 700 higher efficiency is generally achieved with a larger inductor (k) 600 value, which produces a lower switching frequency. For a R ISET 500 given inductor type, however, as inductance is increased 400 DC resistance (DCR) also increases. Higher DCR translates 300 200 into higher copper losses and lower current rating, both 100 of which place an upper limit on the inductance. The 0 recommended range of inductor values for small surface 10 15 20 25 30 35 40 45 50 MAXIMUM LOAD CURRENT (mA) mount inductors as a function of peak current is shown in 3642 F01 Figure 4. The values in this range are a good compromise
Figure 1. R
between the tradeoffs discussed above. For applications
ISET Selection
3642fc 10 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS