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LED6000 数据表(PDF) 24 Page - STMicroelectronics

部件名 LED6000
功能描述  3 A, 61 V monolithic current source with dimming capability
PDF  49 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

LED6000 数据表(HTML) 24 Page - STMicroelectronics

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Application notes - step-down conversion
LED6000
24/49
DocID027777 Rev 2
5
Application notes - step-down conversion
5.1
Input capacitor selection
The input capacitor must be rated for the maximum input operating voltage and the
maximum RMS input current.
Since the step-down converters input current is a sequence of pulses from 0A to IOUT, the
input capacitor must absorb the equivalent RMS current which can be up to the load current
divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these
capacitors must be very high to minimize the power dissipation generated by the internal
ESR, thereby improving system reliability and efficiency.
The RMS input current (flowing through the input capacitor) in step-down conversion is
roughly estimated by:
Equation 7
The actual DC/DC conversion duty cycle, D = VOUT/VIN, is influenced by a few parameters:
Equation 8
where VF is the freewheeling diode forward voltage and VSW the voltage drop across the
internal high-side MOSFET. Considering the range DMIN to DMAX it is possible to determine
the maximum ICIN,RMS flowing through the input capacitor.
The input capacitor value must be dimensioned to safely handle the input RMS current and
to limit the VIN and VCC ramp-up slew rate to 0.5 V/µs maximum, in order to avoid the
device active ESD protections turn-on.

D
D
I
I
OUT
RMS
CIN
1
,
MIN
SW
MAX
IN
F
OUT
MIN
MAX
SW
MIN
IN
F
OUT
MAX
V
V
V
V
D
V
V
V
V
D
,
,
,
,



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