AAT1121
1.5MHz, 250mA Step-Down Converter
Input Capacitor
Input Ripple VPP = 25mV
1
1
CIN =
=
= 1.38µF; use 4.7µF
⎛
⎝
VPP
IO
⎞
⎛
⎝
25mV
0.2A
⎞
· 4
- ESR
·
4
·
FS
- 5mΩ
· 1.5MHz
⎠
⎠
IO
IRMS
=
= 0.1Arms
2
P = esr
·
IRMS2 = 5mΩ
·
(0.1A)2 = 0.05mW
AAT1121 Losses
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN -VO
])
PTOTAL
=
VIN
+ (tsw · F · IO + IQ) · VIN
0.22 · (0.7
Ω
·
1.8V + 0.7Ω
4.2V
· [4.2V - 1.8V])
=
+ (5ns · 1.5MHz · 0.2A + 30µA) · 4.2V = 34.5mW
TJ(MAX) = TAMB + ΘJA · PLOSS = 85°C + (50°C/W) · 34.5mW = 86.7°C
16
1121.2006.04.1.0