Datasheet LTC3632 (Analog Devices) - 10

LTC3632
APPLICATIONS INFORMATION
The basic LTC3632 application circuit is shown on the front average output current for this architecture is limited page of this data sheet. External component selection is to half of the peak current. Therefore, be sure to select determined by the maximum load current requirement and a value that sets the peak current with enough margin begins with the selection of the peak current programming to provide adequate load current under all foreseeable resistor, RISET. The inductor value L can then be determined, operating conditions. followed by capacitors CIN and COUT.
Inductor Selection Peak Current Resistor Selection
The inductor, input voltage, output voltage and peak cur- The peak current comparator has a maximum current limit rent determine the switching frequency of the LTC3632. of 50mA nominally, which results in a maximum aver- For a given input voltage, output voltage and peak current, age current of 25mA. For applications that demand less the inductor value sets the switching frequency when the current, the peak current threshold can be reduced to as output is in regulation. A good first choice for the inductor little as 10mA. This lower peak current allows the use of value can be determined by the following equation: lower value, smaller components (input capacitor, output capacitor and inductor), resulting in lower input supply  V   V  L = OUT   • 1– OUT  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 50mA/I ing equation: PEAK. An additional constraint on the inductor value is the RISET = IPEAK • 21 • 106 LTC3632’s 100ns minimum on-time of the high side switch. where 10mA < IPEAK < 50mA. 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 10mA to 50mA. Shorting the I larger than LMIN, which can be computed as follows: SET pin to ground programs the current limit to 10mA, and leaving it floating VIN(MAX) • tON(MIN) sets the current limit to the maximum value of 50mA. When LMIN = I selecting this resistor value, be aware that the maximum 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 above equation provides the minimum inductor value, 800 higher efficiency is generally achieved with a larger inductor 700 (k) 600 value, which produces a lower switching frequency. For a 500 given inductor type, however, as inductance is increased R ISET 400 DC resistance (DCR) also increases. Higher DCR trans- 300 lates into higher copper losses and lower current rating, 200 both of which place an upper limit on the inductance. The 100 0 recommended range of inductor values for small surface 4 6 8 10 12 14 16 18 20 mount inductors as a function of peak current is shown in MAXIMUM LOAD CURRENT (mA) Figure 4. The values in this range are a good compromise 3632 F01 between the trade-offs discussed above. For applications
Figure 1. RISET Selection
3632fc 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