Datasheet LT3430, LT3430-1 (Analog Devices) - 10

LT3430/LT3430-1
APPLICATIONS INFORMATION
inductor value to achieve a desirable output ripple volt-
Ceramic Output Capacitor
age level. If output ripple voltage is of less importance, An alternative way to further reduce output ripple voltage the subsequent suggestions in Peak Inductor and Fault is to reduce the ESR of the output capacitor by using a Current and EMI will additionally help in the selection of ceramic capacitor. Although this reduction of ESR removes the inductor value. a useful zero in the overall loop response, this zero can Peak-to-peak output ripple voltage is the sum of a triwave be replaced by inserting a resistor (RC) in series with the (created by peak-to-peak ripple current (ILP-P) times ESR) VC pin and the compensation capacitor CC. (See Ceramic and a square wave (created by parasitic inductance (ESL) Capacitors in Applications Information.) and ripple current slew rate). Capacitive reactance is as- sumed to be small compared to ESR or ESL.
Peak Inductor Current and Fault Current
dI To ensure that the inductor will not saturate, the peak V = I ESR ESL RIPPLE (LP P-)( )+( ) inductor current should be calculated knowing the dt maximum load current. An appropriate inductor should where: then be chosen. In addition, a decision should be made whether or not the inductor must withstand continuous ESR = equivalent series resistance of the output capaci- fault conditions. tor If maximum load current is 1A, for instance, a 1A induc- ESL = equivalent series inductance of the output capaci- tor may not survive a continuous 4A overload condition. tor Dead shorts will actually be more gentle on the inductor dI/dt = slew rate of inductor ripple current = VIN/L because the LT3430/LT3430-1 have frequency and current Peak-to-peak ripple current (I limit foldback. LP-P) through the inductor and into the output capacitor is typically chosen to be
Table 2
between 20% and 40% of the maximum load current. It
VENDOR/ VALUE I DCR HEIGHT
is approximated by:
DC PART NO. (µH) (Amps) (Ohms) (mm)
V V – V ( )( )
Sumida
I OUT IN OUT LP-P = V f L ( )( )( ) CDRH104R-150 15 3.6 0.050 4 IN CDRH104R-220 22 2.9 0.073 4 Example: with VIN = 40V, VOUT = 5V, L = 22µH, ESR = CDRH104R-330 33 2.3 0.093 4 0.080Ω and ESL = 10nH, output ripple voltage can be CDRH124-220 22 2.9 0.066 4.5 approximated as follows: CDRH124-330 33 2.7 0.097 4.5 ( )5(40− )5 CDRH127-330 33 3.0 0.065 8 I = 0 9 . 9A P-P ( CDRH127-470 47 2.5 0.100 8 40)( − 22 • 10 6)(200 •103) = CEI122-220 22 2.3 0.085 8 dI Coiltronics = 40 = 106 • 1.8 dt − 22 • 10 6 UP3B-330 33 3 0.069 6.8 UP3B-470 47 2.4 0.108 6.8 V = 0 99 . A −9 6 RIPPLE ( )(0.08)+(10•10 )(10 )(1.8) UP4B-680 68 4.3 0.120 7.9 = 0 079 . + 0.018 = mV 97 P-P Coilcraft DO3316P-153 15 3 0.046 5.2 To reduce output ripple voltage further requires an increase in the inductor value with the trade-off being a physically DO5022p-683 68 3.5 0.130 7.1 larger inductor with the possibility of increased component height and cost. 34301fa 10