Datasheet LTC3631 (Analog Devices) - 10

LTC3631
applicaTions inForMaTion
The basic LTC3631 application circuit is shown on the front maximum average output current for this architecture is page of this data sheet. External component selection is limited to half of the peak current. Therefore, be sure to determined by the maximum load current requirement and select a value that sets the peak current with enough mar- begins with the selection of the peak current programming gin 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 LTC3631. of 225mA nominally, which results in a maximum aver- For a given input voltage, output voltage and peak current, age current of 112mA. 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 50mA. 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 225mA/I ing equation: PEAK. An additional constraint on the inductor value is the RISET = IPEAK • 4.5 • 106 LTC3631’s 100ns minimum on-time of the high side switch. where 50mA < IPEAK < 225mA. 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 range of 50mA to 225mA. Shorting the I is larger than LMIN, which can be computed as follows: SET pin to ground programs the current limit to 50mA, and leaving it floating VIN(MAX) • tON(MIN) sets the current limit to the maximum value of 225mA. 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 above equation provides the minimum inductor value, 800 700 higher efficiency is generally achieved with a larger inductor 600 value, which produces a lower switching frequency. For a (kΩ) 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 20 30 40 50 60 70 80 90 100 mount inductors as a function of peak current is shown in MAXIMUM LOAD CURRENT (mA) 3631 F01 Figure 4. The values in this range are a good compromise
Figure 1. R
between the trade-offs discussed above. For applications
ISET Selection
3631fe 10 For more information www.linear.com/LTC3631 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Related Parts