NEC LCD1550X Service Manual page 39

MB3778PFV-G-BND-EF(IC608) Study of DC/DC step down circuit design
Input voltage V 12V
IN
C604
IC608
MB3778PFV
OSC
2
1
15KΩ
330pF
• Output voltage V
OUT
• Output current I
OUT
• Ripple current ∆I
L
• Choke coil L = (V
IN
L610 (Ch1) =(12V - 0.038Ω • 1.6A - 5.25V) • 5.25V / 0.146Ap-p • 200000Hz • 12V
=100µH
(Sagami Elec. product 7B12N: Tolerable current 1.7A, direct-current resistance 0.17Ω)
L603 (Ch2) = (12V - 0.038Ω • 0.8A - 3.40V) • 3.40V / 0.121Ap-p • 200000Hz • 12V
=100µH
(Taiyo yuden product NO8DP [A Typ.]: Tolerable current 1.05A, direct-current resistance 0.35Ω)
• Maximum power consumption P = (5.25V x 1.6A + 3.40V x 0.8A)/ efficiency 0.8
• Maximum input current I
Input fuse selection: 1.6A part x 97% x 75% = 1.16A > 1.15A >0.7A
* The above maximum input current is calculated from the DC/DC maximum output current
specifications, and is not the input current value that flows at the maximum load.
The DC/DC maximum output current value has a margin over the maximum load current value,
so the actual maximum input current is approx. 0.7A.
IC607
V
CC
9
10
13
IC609
1.23V
7
4
1.23V
(specification) V
OUT
V
OUT
(design value)
= (V
REF
V
OUT
V
OUT
(specifications) I
OUT
I
OUT
∆I
(specifications)
L
∆I
L
- R • I - V ) • V
DS (ON) OUT OUT
= 13.9W
* 80% efficiency is value generally used for DC/DC design
= 13.9W / 12V
IN
= 1.15A
Temperature derating Rating derating
L610
C641
L603
C652
(Ch1) = 5.25V (Typ.)
(Ch2) = 3.40V (Typ.)
/2) • (1+ R1/R2)
(Ch1) = (2.46V/2) • (1+(5.6kΩ+330Ω)/1.8kΩ) = 5.28V
(Ch2) = (2.46V/2) • (1+(5.6kΩ+270Ω)/3.3kΩ) = 3.42V
(Ch1) = 1.6A (MAX.)
(Ch2) = 0.8A (MAX.)
(Ch1) = 0.146Ap-p
(Ch2) = 0.121Ap-p
/ ∆I
• f • V
OUT L s IN
1 - 33
Output voltage V
OUT
330Ω
5.6kΩ
1.8kΩ
Output voltage V
OUT
270Ω
5.6kΩ
3.3kΩ
* V = 2.46V±2%
REF
Set to 10% of I (Ch1)
OUT
Set to 15% of I (Ch2)
OUT
* Generally set to 30% or less
(Ch1) 5.25V
(Ch2) 3.40V
OK