Datasheet LT1372, LT1377 (Analog Devices) - 9

LT1372/LT1377
U U W U APPLICATIO S I FOR ATIO Table 1. Surface Mount Solid Tantalum Capacitor
0.3(V
ESR and Ripple Current
I IN)(VOUT – VIN) RIPPLE = (f)(L)(VOUT)
E CASE SIZE ESR (MAX
Ω
) RIPPLE CURRENT (A)
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 f = 500kHz Switching frequency (LT1372) or, AVX TAJ 0.7 to 0.9 0.4 1MHz Switching frequency (LT1377)
D CASE SIZE
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 The input capacitor can see a very high surge current when AVX TAJ 0.9 to 2.0 0.36 to 0.24 a battery or high capacitance source is connected “live”
C CASE SIZE
and solid tantalum capacitors can fail under this condition. AVX TPS 0.2 (Typ) 0.5 (Typ) Several manufacturers have developed a line of solid AVX TAJ 1.8 to 3.0 0.22 to 0.17 tantalum capacitors specially tested for surge capability
B CASE SIZE
(AVX TPS series, for instance), but even these units may AVX TAJ 2.5 to 10 0.16 to 0.08 fail if the input voltage approaches the maximum voltage rating of the capacitor. AVX recommends derating capaci- Many engineers have heard that solid tantalum capacitors tor voltage by 2:1 for high surge applications. Ceramic and are prone to failure if they undergo high surge currents. aluminum electrolytic capacitors may also be used and This is historically true and type TPS capacitors are have a high tolerance to turn-on surges. specially tested for surge capability, but surge ruggedness is not a critical issue with the output capacitor. Solid
Ceramic Capacitors
tantalum capacitors fail during very high turn-on surges, Higher value, lower cost ceramic capacitors are now which do not occur at the output of regulators. High becoming available in smaller case sizes. These are tempt- discharge surges, such as when the regulator output is ing for switching regulator use because of their very low dead shorted, do not harm the capacitors. ESR. Unfortunately, the ESR is so low that it can cause Single inductor boost regulators have large RMS ripple loop stability problems. Solid tantalum capacitor ESR current in the output capacitor, which must be rated to generates a loop “zero” at 5kHz to 50kHz that is instrumen- handle the current. The formula to calculate this is: tal in giving acceptable loop phase margin. Ceramic ca- pacitors remain capacitive to beyond 300kHz and usually Output Capacitor Ripple Current (RMS) resonate with their ESL before ESR becomes effective. DC They are appropriate for input bypassing because of their I 1 – DC high ripple current ratings and tolerance of turn-on surges. RIPPLE (RMS) = IOUT Linear Technology plans to issue a Design Note on the use VOUT – VIN of ceramic capacitors in the near future. = IOUT VIN
Output Diode Input Capacitors
The suggested output diode (D1) is a 1N5818 Schottky or The input capacitor of a boost converter is less critical due its Motorola equivalent, MBR130. It is rated at 1A average to the fact that the input current waveform is triangular and forward current and 30V reverse voltage. Typical forward does not contain large squarewave currents as is found in voltage is 0.42V at 1A. The diode conducts current only the output capacitor. Capacitors in the range of 10µF to during switch off time. Peak reverse voltage for boost 100µF with an ESR of 0.3Ω or less work well up to full 1.5A converters is equal to regulator output voltage. Average switch current. Higher ESR capacitors may be acceptable forward current in normal operation is equal to output at low switch currents. Input capacitor ripple current for current. boost converter is : 9