Datasheet LT1931, LT1931A (Analog Devices) - 5

LT1931/LT1931A
U U W U APPLICATIO S I FOR ATIO LT1931A AND LT1931 DIFFERENCES:
core losses at frequencies above 1MHz are much lower for ferrite cores than for powdered-iron units. When using
Switching Frequency
coupled inductors, choose one that can handle at least 1A The key difference between the LT1931A and LT1931 is of current without saturating, and ensure that the inductor the faster switching frequency of the LT1931A. At 2.2MHz, has a low DCR (copper-wire resistance) to minimize I2R the LT1931A switches at nearly twice the rate of the power losses. If using uncoupled inductors, each inductor LT1931. Care must be taken in deciding which part to use. need only handle one-half of the total switch current so The high switching frequency of the LT1931A allows that 0.5A per inductor is sufficient. A 4.7µH to 15µH smaller cheaper inductors and capacitors to be used in a coupled inductor or a 15µH to 22µH uncoupled inductor given application, but with a slight decrease in efficiency will usually be the best choice for most LT1931 designs. and maximum output current when compared to the For the LT1931A, a 2.2µH to 4.7µH coupled inductor or a LT1931. Generally, if efficiency and maximum output 3.3µH to 10µH uncoupled inductor will usually suffice. In current are critical, the LT1931 should be used. If applica- certain applications such as the “Charge Pump” inverting tion size and cost are more important, the LT1931A will be DC/DC converter, only a single inductor is used. In this the better choice. In many applications, tiny inexpensive case, the inductor must carry the entire 1A switch current. chip inductors can be used with the LT1931A, reducing
Table 1. Recommended Inductors—LT1931
solution cost.
L Size PART (
µ
H) (L
×
W
×
H) mm VENDOR Duty Cycle
CLS62-100 10 6.8 × 6.6 × 2.5 Sumida CR43-150 15 4.5 × 4.0 × 3.2 (847) 956-0666 The maximum duty cycle (DC) of the LT1931A is 75% CR43-220 22 www.sumida.com compared to 84% for the LT1931. The duty cycle for a CTX10-1 10 8.9 × 11.4 × 4.2 Coiltronics given application using the dual inductor inverting topol- CTX15-1 15 (407) 241-7876 ogy is given by: www. coiltronics.com LQH3C100K24 10 3.2 × 2.5 × 2.0 Murata | V | LQH4C150K04 15 (404) 436-1300 DC OUT = www.murata.com | V | + | V IN OUT |
Table 2. Recommended Inductors—LT1931A
For a 5V to –5V application, the DC is 50% indicating that the LT1931A can be used. A 5V to –16V application has a
L Size PART (
µ
H) (L
×
W
×
H) mm VENDOR
DC of 76.2% making the LT1931 the right choice. The ELJPC3R3MF 3.3 2.5 × 2.0 × 1.6 Panasonic LT1931A can still be used in applications where the DC, as ELJPC4R7MF 4.7 (408) 945-5660 calculated above, is above 75%. However, the part must www.panasonic.com be operated in the discontinuous conduction mode so that CLQ4D10-4R71 4.7 7.6 × 4.8 × 1.8 Sumida the actual duty cycle is reduced. CLQ4D10-6R82 6.8 (847) 956-0666 www.sumida.com LB20164R7M 4.7 2.0 × 1.6 × 1.6 Taiyo Yuden
INDUCTOR SELECTION
LB20163R3M 3.3 (408) 573-4150 www.t-yuden.com Several inductors that work well with the LT1931 are listed in Table 1 and those for the LT1931A are listed in Table 2. LQH3C4R7K24 4.7 3.2 × 2.5 × 2.0 Murata LQH4C100K24 10 (404) 436-1300 Besides these, there are many other inductors that can be www.murata.com used. Consult each manufacturer for detailed information 1Use drawing #5382-T039 and for their entire selection of related parts. Ferrite core 2Use drawing #5382-T041 inductors should be used to obtain the best efficiency, as 1931fa 5