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LT3437IDD Datasheet(PDF) 11 Page - Linear Technology |
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LT3437IDD Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3437 11 3437f APPLICATIO S I FOR ATIO may still affect the proper operation of the regulator. A filter capacitor CF in parallel with the RC/CC network, along with a small feedforward capacitor CFB, is suggested to control possible ripple at the VC pin. The LT3437 can be stabilized using a 100 µF ceramic output capacitor and VC component values of CC = 1500nF, RC = 25k, CF = 330pF and CFB =27pF. OUTPUT RIPPLE VOLTAGE Figure 3 shows a typical output ripple voltage waveform for the LT3437. Ripple voltage is determined by the impedance of the output capacitor and ripple current through the inductor. Peak-to-peak ripple current through the inductor into the output capacitor is: I VV V VL f OUT IN OUT IN P-P = () ()( )( ) – For high frequency switchers the ripple current slew rate is also relevant and can be calculated from: di dt V L IN = Peak-to-peak output ripple voltage is the sum of a triwave created by peak-to-peak ripple current times ESR and a square wave created by parasitic inductance (ESL) and ripple current slew rate. Capacitive reactance is assumed to be small compared to ESR or ESL. V I ESR ESL di dt RIPPLE = ()( )+() P-P Example: with VIN = 12V, VOUT = 3.3V, L = 100µH, ESR = 0.075 Ω, ESL = 10nH: IP-P = ()( ) () − ()( ) = == 33 12 33 12 100 6 200 3 0 120 12 100 6 012 6 .– . . – . ee A di dt e e VRIPPLE = (0.120A)(0.075) + (10e – 9)(0.12e6) = 0.009 + 0.0012 = 10.2mVP-P Table 2. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E CASE SIZE ESR MAX ( Ω) RIPPLE CURRENT (A) AVX TPS 0.1 to 0.3 0.7 to 1.1 D CASE SIZE AVX TPS 0.1 to 0.3 0.7 to 1.1 C CASE SIZE AVX TPS 0.2 0.5 Many engineers have heard that solid tantalum capacitors are prone to failure if they undergo high surge currents. This is historically true, and type TPS capacitors are specially tested for surge capability, but surge ruggedness is not a critical issue with the output capacitor. Solid tantalum capacitors fail during very high turn-on surges which do not occur at the output of regulators. High discharge surges, such as when the regulator output is dead shorted, do not harm the capacitors. Unlike the input capacitor RMS, ripple current in the output capacitor is normally low enough that ripple cur- rent rating is not an issue. The current waveform is triangular with a typical value of 30mARMS. The formula to calculate this is: Output capacitor ripple current (RMS) I VV V Lf V RIPPLE RMS OUT IN OUT IN () .– = ()( ) ()( )( ) = 029 12 IP-P CERAMIC CAPACITORS Higher value, lower cost ceramic capacitors are now becoming available. They are generally chosen for their good high frequency operation, small size and very low ESR (effective series resistance). Low ESR reduces output ripple voltage but also removes a useful zero in the loop frequency response, common to tantalum capacitors. To compensate for this, a resistor RC can be placed in series with the VC compensation capacitor CC (Figure 10). Care must be taken, however, since this resistor sets the high frequency gain of the error amplifier, including the gain at the switching frequency. If the gain of the error amplifier is high enough at the switching frequency, output ripple voltage (although smaller for a ceramic output capacitor) |
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