Microchip Technology MCP1631RD-MCC2 Hoja De Datos
Design Example
© 2009 Microchip Technology Inc.
DS51791A-page 48
• R
Artificial Ramp
=
τ
Artificial Ramp
/C
Artificial Ramp
= 15.13 µs/1500 pF
= 10 k
Ω (max), use around 5.6 kΩ to 9.1 kΩ for tolerances
An R value of 5.6 k
Ω was used for the Eval Board.
D.1.4
Coupling Capacitor:
• SW
ON
, D = 25%, t = 0.5 µs
I
RIPPLE
= t
ON
* 2V
IN
/4L = 0.5 µs * (2)(6V)/(4*10 µH) = 0.150A
I
1
= I
OUT
– ½ I
RIPPLE
= 1.5A – 0.075A = 1.425A
I
2
= I
OUT
+ ½ I
RIPPLE
= 1.5A + 0.075A = 1.575A
I
RMS_ON
= SQRT(D * (I
1
2
+ I
1
* I
2
+ I
2
2
)/3) (for a trapezoidal waveform)
= SQRT(0.25 * (1.425
2
+ 1.425 * 1.575 + 1.575
2
)/3
= 0.750A
• SW
OFF
, D = 75%, t = 1.5 µs
I
RIPPLE
= t
OFF
* 2V
IN
/4L = 1.5 µs * (2)(6V)/(4*10 µH) = 0.450A
I
1
= -I
IN_WC
– ½ I
RIPPLE
= -2.06A – 0.225A = -2.285A
I
2
= -I
IN_WC
+ ½ I
RIPPLE
= -2.06A + 0.225A = -1.835A
I
RMS_OFF
= SQRT(D * (I
1
2
+ I
1
* I
2
+ I
2
2
)/3)
= SQRT(0.75 * [-2.285
2
+ (-2.285 * (-1.835)) + (-1.835)
2
]/3)
= 1.79A
- I
RMS
= SQRT (I
RMS_ON
2
+ I
RMS_OFF
2
)
= SQRT (0.750
2
+ 1.79
2
)
= 1.94A
• Worst-Case Capacitor Current is 1.94A.
Choose a capacitor that is rated for at least the worst-case current.