Datasheet

www.ti.com
Voltage Setting
FB
OUT1 FB
V R2
R4
V V
´
=
-
(32)
FB
OUT2 FB
V R9
R7
V V
´
=
-
(33)
Compensation Capacitors
f
ESR(zero)
1
2 C ESR
=
´ p ´ ´
(34)
f
f
ZERO(desired)
ESR(zero)
R4
R5
1
=
æ ö
æ ö
ç ÷
-
ç ÷
ç ÷
ç ÷
è ø
è ø
(35)
EQ
1
R R5
1 1
R2 R4
= +
æ ö
æ ö æ ö
+
ç ÷
ç ÷ ç ÷
è ø è ø
è ø
(36)
f
EQ ESR(zero)
1
C8
2 R
=
´ p ´ ´
(37)
TPS54283 , , TPS54286
SLUS749C – JULY 2007 – REVISED OCTOBER 2007
The primary feedback divider resistors (R2, R9) from VOUT to FB should be between 10 k Ω and 50 k Ω to
maintain a balance between power dissipation and noise sensitivity. For this design, 20 k Ω is selected.
The lower resistors, R4 and R7 are found using the following equations.
• R2 = R9 = 20 k Ω
• V
FB
= 0.80 V
• R4= 3.80 k Ω (3.83 k Ω standard value is used)
• R7= 6.40 k Ω (6.34 k Ω standard value is used)
Checking the ESR zero of the output capacitors:
• C = 100 µ F
• ESR = 400 m Ω
• ESR
(zero)
= 3980 Hz
Since the ESR zero of the main output capacitor is less than 20 kHz, an R-C filter is added in parallel with R4
and R7 to compensate for the electrolytic capacitors' ESR and add a zero about 40 kHz.
• f
ESR(zero)
= 4 kHz
• f
ESR(desired)
= 40 kHz
• R4 = 3.83 k Ω
• R5 = 424 Ω (422 Ω selected)
• R7 = 6.34 k Ω
• R8 = 702 Ω (698 Ω selected)
• R2 = R9 = 20 k Ω
• REQ1 = 3.63 k Ω
• REQ2 = 5.51 k Ω
• C8 = 10.9 nF (10 nF selected)
• C15 = 7.22 nF (6800 pF selected)
36 Submit Documentation Feedback Copyright © 2007, Texas Instruments Incorporated
Product Folder Link(s): TPS54283 TPS54286