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C
SS
= 0.47 PF
C
SS
=
= = 540 nF
(40 ms - 29.2 ms)
(t
TSU
- t
SU-SS-BASE
)
20 k: 20 k:
V
O
I
LED
t
SU-SS-BASE
= 168: x C9 + 28 k:x C10 + x C
O
t
SU-SS-BASE
= 29.2 ms
21V
1A
t
SU-SS-BASE
= 168: x 2.2 PF + 28 k:x 1.0 PF + x 40 PF
V
O
I
LED
t
SU
= 168: x C9 + 36 k:x C10 + x C
O
t
SU
= 37.2 ms
21V
1A
t
SU
= 168: x 2.2 PF + 36 k:x 1.0 PF + x 40 PF
R11 = 12.4 k:
R17 = 499 k:
= 51.1V
12.4 k:
1.24V x (12.4 k: + 499 k:)
V
TURN-OFF
=
R11
1.24V x (R11 + R17)
V
TURN-OFF
=
R11 = =
= 12.5 k:
V
TURN-OFF
- 1.24V
1.24V x R17
1.24V x 499 k:
50V - 620 mV
V
HYSO
= R17 x 20 PA = 499 k: x 20 PA = 9.98V
R17 =
=
= 500 k:
V
HYSO
10V
20 PA20 PA
R4 = 17.4 k:
R13 = 10 k:
R5 = 1.43 k:
www.ti.com Design Procedure
The chosen components from step 13 are:
(53)
8.15 Output OVLO
Solve for R
OV2
:
(54)
The closest standard resistor is 499 kΩ therefore V
HYSO
is:
(55)
Solve for R
OV1
:
(56)
The closest standard resistor is 15.8 kΩ making V
TURN-OFF
:
(57)
The chosen components from step 14 are:
(58)
8.16 Soft-Start
Solve for t
SU
:
(59)
If t
SU
is less than t
TSU
, solve for t
SU-SS-BASE
:
(60)
Solve for C
SS
:
(61)
The chosen component from step 15 is:
(62)
13
SNVA397A–August 2009–Revised May 2013 AN-1967 LM3424 Buck-Boost Evaluation Board
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