Datasheet LTC3620 (Analog Devices) - 10

LTC3620
APPLICATIONS INFORMATION Choosing an Inductor
The part is optimized to get 35mA peaks for VIN = 3.6V and VOUT = 1.1V with an 18μH inductor. If the falling slope is There are a number of different values, sizes and brands too steep the NFET will continue to conduct shortly after of inductors that will work well with this part. Table 1 has a the inductor current reaches zero, causing a small reverse number of recommended inductors, though there are many current. This means the net power delivered with every other manufacturers and devices that may also be suitable. pulse will decrease. To mitigate this problem the inductor Consult each manufacturer for more detailed information can be resized. Table 2 shows recommended inductors and and for their entire selection of related parts. output capacitors for commonly used output voltages.
Table 1: Representative Surface Mount Inductors Table 2. Recommended Inductor and Output Capacitor Sizes for MAX DC Different VOUT PART VALUE CURRENT W
×
L
×
H V VENDOR NUMBER (μH) DCR (Ω) (mA) (mm3) OUT (V) L (μH) COUT (μF)
0.9 15 2.2 Taiyo CBMF1608T 22 ±10% 1.3 Max 70 0.8 × 1.6 × 0.8 Yuden 1.1 22 1 Murata LQH2MC_02 18 ±20% 1.8 ±30% 190 1.6 × 2 × 0.9 1.1 (LTC3620-1) 22 2.2 22 ±20% 2.1 ±30% 185 1.8 33 2.2 Würth 744028220 22 ±30% 1.48 Max 270 2.8 × 2.8 × 1.1 2.5 47 4.7 Electronics Coilcraft LPS3010 18 ±20% 1.0 Max 380 2.95 × 2.95 × 0.9 Because the rising dI/dt decreases for increased VOUT 22 ±20% 1.2 Max 320 and increased L, the inductor current peaks will decrease, causing the maximum load current to decrease as well. There is a trade-off between physical size and effi ciency; Figure 2 shows typical maximum load current versus The inductors in Table 1 are shown because of their small output voltage. footprints, choose larger sized inductors with less core loss and lower DCR to maximize effi ciency. 20 TA = 25°C 19 The ideal inductor value will vary depending on which 18 characteristics are most critical to the designer. Use the 17 equations and recommendations in the next sections to help 16 you fi nd the correct inductance value for your design. 15 14
Avoiding Audio Range Switching
13 In order to best avoid switching in the audio range at the 12 MAXIMUM LOAD CURRENT (mA) lowest possible load current, the minimum frequency 11 should be set as low as is acceptable, and the inductor 10 0.6 1.1 1.6 2.1 2.6 value should be minimized. For a 1.1V output the smallest OUTPUT VOLTAGE (V) 3620 F02 recommended inductor value is 15μH.
Figure 2. Maximum Output Current vs VOUT , VIN = 3.6V Adjusting for VOUT Output Voltage Ripple
The inductor current peak and zero crossing are dependent The quantity of charge transferred from VIN to VOUT per on the dI/dt. The equations for the rising and falling slopes switching cycle is directly proportional to the inductor are as follows: value. Consequently, the output voltage ripple is directly Rising dI/dt = (VIN-VOUT)/L proportional to the inductor value, and the switching frequency for a given load is inversely proportional to the Falling dI/dt = VOUT/L inductor value. For a given load current, higher switching frequency will typically lower the effi ciency because of the 3620fa 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 APPLICATIONS RELATED PARTS