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SC1470EVB Datasheet(PDF) 11 Page - Semtech Corporation |
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SC1470EVB Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 25 page  11 2005 Semtech Corp. www.semtech.com SC1470 POWER MANAGEMENT V BAT via RtON, converting this to a current. This current is used to charge an internal 3.3pF capacitor to V OUT. The equations above reflect this along with any internal components or delays that influence t ON. For our example we select R tON = 1MΩ: t ON_VBAT(MIN) = 563ns and tON_VBAT(MAX) = 255ns f SW_VBAT(MIN) = 266kHz and fSW_VBAT(MAX) = 235kHz Now that we know t ON we can calculate suitable values for the inductor. To do this we select an acceptable inductor ripple current. The calculations below assume 50% of I OUT which will give us a starting place. () ()H I 5 . 0 t V V L OUT ) MIN ( VBAT _ ON OUT ) MIN ( BAT ) MIN ( VBAT • • − = and () () H I 5 . 0 t V V L OUT ) MAX ( VBAT _ ON OUT ) MAX ( BAT ) MAX ( VBAT • • − = For our example: L VBAT(MIN) = 1.3µH and LVBAT(MAX) = 1.6µH We will select an inductor value of 2.2µH to reduce the ripple current, which can be calculated as follows: () P P ) MIN ( VBAT _ ON OUT ) MIN ( BAT ) MIN ( VBAT _ RIPPLE A L t V V I − • − = and () P P ) MAX ( VBAT _ ON OUT ) MAX ( BAT ) MAX ( VBAT _ RIPPLE A L t V V I − • − = For our example: I RIPPLE_VBAT(MIN) = 1.74AP-P and IRIPPLE_VBAT(MAX) = 2.18AP-P From this we can calculate the minimum inductor current rating for normal operation: ) MIN ( ) MAX ( VBAT _ RIPPLE ) MAX ( OUT ) MIN ( INDUCTOR A 2 I I I + = For our example: I INDUCTOR(MIN) = 7.1A(MIN) Next we will calculate the maximum output capacitor equivalent series resistance (ESR). This is determined by calculating the remaining static and transient tolerance allowances. Then the maximum ESR is the smaller of the calculated static ESR (R ESR_ST(MAX)) and transient ESR (R ESR_TR(MAX)): () Ohms I 2 ERR ERR R ) MAX ( VBAT _ RIPPLE DC ST ) MAX ( ST _ ESR • − = Where ERR ST is the static output tolerance and ERRDC is the DC error. The DC error will be 1% plus the tolerance of the feedback resistors, thus 2% total for 1% feedback resistors. For our example: ERR ST = 48mV and ERRDC = 24mV, therefore R ESR_ST(MAX) = 22mΩ () Ohms 2 I I ERR ERR R ) MAX ( VBAT _ RIPPLE OUT DC TR ) MAX ( TR _ ESR + − = Where ERR TR is the transient output tolerance. Note that this calculation assumes that the worst case load transient is full load. For half of full load, divide the I OUT term by 2. For our example: ERR TR = 96mV and ERRDC = 24mV, therefore R ESR_TR(MAX) = 10.2mΩ for a full 6A load transient We will select a value of 12.5m Ω maximum for our design, which would be achieved by using two 25m Ω output capacitors in parallel. Note that for constant-on converters there is a minimum ESR requirement for stability which can be calculated as follows: |
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