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LT1507IS8-3.3 Datasheet(PDF) 11 Page - Linear Technology |
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LT1507IS8-3.3 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1507 APPLICATIONS INFORMATION appropriate for input bypassing because of their high ripple current ratings and tolerance of turn-on surges. OUTPUT RIPPLE VOLTAGE Ripple voltage is determined by the high frequency imped- ance of the output capacitor and ripple current through the inductor. Ripple current is triangular (continuous mode) with a peak-to-peak value of: I VV V VL f OUT IN OUT IN P-P = − ()( ) ()( )( ) Output ripple voltage is also triangular with peak-to-peak amplitude of: VRIPPLE = (IP–P)(ESR) (peak-to-peak) Example: with VIN = 5V, VOUT = 3.3V, L = 5µH, ESR = 0.1Ω; I VA mV RIPPLE P-P P-P P-P = − = =Ω = − (. )( . ) . (. )( . ) 33 5 3 3 5 5 10 500 10 045 045 0 1 45 63 MAXIMUM OUTPUT LOAD CURRENT Maximum load current will be less than the 1.5A rating of the LT1507, especially with lower inductor values. Induc- tor ripple current must be taken into account as well as reduced switch current at high duty cycles. Maximum switch current rating (IP) of the LT1507 is 1.5A up to 50% duty cycle (DC), decreasing to 1.35A at 80% duty cycle, shown graphically in Typical Performance Characteristics and as a formula below. Current rating decreases with duty cycle because the LT1507 has internal slope com- pensation to prevent current mode subharmonic switch- ing. For more details on subharmonic oscillation read Application Note 19. Peak guaranteed switch current (IP) is found from: IA V IA V V V P IN P OUT IN IN =≤ =− ≥ 15 05 175 05 05 .. . .( ) . for V for V OUT OUT Table 2. Surface Mount Solid Tantalum Capacitor ESR and Ripple Current E CASE SIZE ESR (MAX Ω) RIPPLE CURRENT (A) AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 AVX TAJ 0.7 to 0.9 0.4 D CASE SIZE AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 AVX TAJ 0.9 to 2.0 0.36 to 0.24 C CASE SIZE AVX TPS 0.2 (Typ) 0.5 (Typ) AVX TAJ 1.8 to 3.0 0.22 to 0.17 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 rugged- ness 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 200mA RMS. The formula to calculate this is: Output Capacitor Ripple Current (RMS) I RMS VV V Lf V RIPPLE OUT IN OUT IN () .( )( – ) ()( )( ) = 029 Ceramic Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. These are tempt- ing for switching regulator use because of their very low ESR. Unfortunately, the ESR is so low that it can cause loop stability problems when ceramic is used for the output capacitor. Solid tantalum capacitor ESR generates a loop “zero” at 5kHz to 50kHz that is instrumental in giving acceptable loop phase margin. Ceramic capacitors remain capacitive to beyond 300kHz and usually resonate with their ESL before ESR becomes effective. They are |
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