NEC LCD1550X Service Manual page 41

PQ1CZ21H2ZP(IC611) Study of DC/DC step down circuit design
Input voltage V 12V
IN
C656
• Output voltage V
OUT
• Output current I
OUT
• Ripple current ∆I
L
• Choke coil L = (V - V
IN
L618 =(12V - 0.9V - 5.70V) • 5.70V / 0.171Ap-p • 100000Hz • 12V
=150µH
(Taiyo yuden product NO6DB 151K : Tolerable current 0.48A, direct-current resistance 1.0Ω)
• Maximum power consumption P = (5.70V x 0.2A) / efficiency 0.8
• Maximum input current I
• Output smoothing capacitor C657 : 16V_100M-M
ESR =1.00Ω MAX / 20˚C, 100kHz
Tolerable ripple current = 140mArms / 105˚C, 100kHz
* Tolerable ripple current frequency compensation coefficient = 1
5.5V output system
Ripple voltage (specifications)
Ripple voltage = ∆I
Ripple current = ∆I
• Input capacitor C656 : 16V_100M-M
ESR = 1.00Ω MAX / 20˚C, 100kHz
Tolerable ripple current = 140mArms / 105˚C, 100kHz
Ripple voltage =
=
=0.50Arms
• Oscillation frequency = 100kHz
2
1
IC611
PQ1CZ21H2ZP
4
(specification) V = 5.70V (Typ.)
OUT
(design value)
= V
REF
V
OUT
(specifications) I
OUT
∆I
(specifications) = 0.171Ap-p
L
/ ∆I
- V ) • V
sat OUT OUT
= 1.43W
* 80% efficiency is value generally used for DC/DC design
= 1.43W / 12V
IN
= 0.12A
<
200mVp-p
=
x ESR (1.00Ω) = 0.171Ap-p x 1.00Ω = 0.171Vp-p
L
/2 3 = 0.049Arms
L
F55(CP182P093A41)
* Tolerable ripple current frequency compensation coefficient = 1
D (1-D )
ON ON
0.48 (1-0.48)
< Tolerable ripple current (140mArms)
=
L618
750Ω
C657
5.6kΩ
1.8kΩ
• (1+ R1/R2)
= 1.26V • 1+(5.6kΩ+750Ω)/1.8kΩ) = 5.71V
= 0.200A (MAX.)
• f • V
* V : internal switch's saturation voltage = 0.9V
L s IN sat
F55 (CP182P093A41)
< =
Tolerable ripple current (140mArms)
D = V / V = 5.7V / 12V =0.48
ON OUT IN
1 - 35
Output voltage V (Ch1) 5.70V
OUT
* V = 1.26V±2%
REF
<
200mVp-p
=
OK
OK
OK