Datasheet LT3431 (Analog Devices) - 10

LT3431
U U W U APPLICATIO S I FOR ATIO
importance, the subsequent suggestions in Peak Induc- physically larger inductor with the possibility of increased tor and Fault Current and EMI will additionally help in the component height and cost. selection of the inductor value.
Ceramic Output Capacitor
Peak-to-peak output ripple voltage is the sum of a triwave An alternative way to further reduce output ripple voltage (created by peak-to-peak ripple current (ILP-P) times ESR) is to reduce the ESR of the output capacitor by using a and a square wave (created by parasitic inductance (ESL) ceramic capacitor. Although this reduction of ESR re- and ripple current slew rate). Capacitive reactance is moves a useful zero in the overall loop response, this zero assumed to be small compared to ESR or ESL. can be replaced by inserting a resistor (RC) in series with dI the VC pin and the compensation capacitor CC. (See V = I ESR - + ESL ( )( ) ( ) RIPPLE LP P Σ Ceramic Capacitors in Applications Information.) dt
Peak Inductor Current and Fault Current
where: To ensure that the inductor will not saturate, the peak ESR = equivalent series resistance of the output inductor current should be calculated knowing the maxi- capacitor mum load current. An appropriate inductor should then ESL = equivalent series inductance of the output be chosen. In addition, a decision should be made whether capacitor or not the inductor must withstand continuous fault dI/dt = slew rate of inductor ripple current = V conditions. IN/L Peak-to-peak ripple current (I If maximum load current is 1A, for instance, a 1A inductor LP-P) through the inductor and into the output capacitor is typically chosen to be may not survive a continuous 4A overload condition. Dead between 20% and 40% of the maximum load current. It is shorts will actually be more gentle on the inductor because approximated by: the LT3431 has frequency and current limit foldback. Peak inductor and switch current can be significantly V V – V ( )( ) OUT IN OUT higher than output current, especially with smaller induc- ILP-P = V f L ( )( )( ) tors and lighter loads, so don’t omit this step. Powdered IN iron cores are forgiving because they saturate softly, Example: with V whereas ferrite cores saturate abruptly. Other core mate- IN = 12V, VOUT = 5V, L = 10µH, ESR = 0.080Ω and ESL = 10nH, output ripple voltage can be rials fall somewhere in between. The following formula approximated as follows: assumes continuous mode of operation, but errs only slightly on the high side for discontinuous mode, so it can ( ) be used for all conditions. 5 (12 − ) 5 I = 0.5 A P- P ( ) 8 ( ) ( )( – - ) 12 ( − 10 •10 6)(500•103) = I V V V OUT IN OUT I = I LP P + = I PEAK OUT OUT + 2 ( )2( )( )( ) dI V f L Σ = 12 = IN 106 •1.2 dt − 10 •10 6
EMI
V = 0.5 A ( 8 )(0. ) −9 6 RIPPLE 08 + (10•10 )(10 )(1.2) Decide if the design can tolerate an “open” core geometry = 0.046 + 0.012 = mV 58 like a rod or barrel, which have high magnetic field P- P radiation, or whether it needs a closed core like a toroid to To reduce output ripple voltage further requires an in- prevent EMI problems. This is a tough decision because crease in the inductor value with the trade-off being a the rods or barrels are temptingly cheap and small and sn3431 3431fs 10