Datasheet

R
OV1
=
R1.24V
OV2
x
Vm620V
OFFTURN
-
-
R
OV1
=
1.24VV
OFFTURN
-
-
R1.24V
OV2
x
R
OV2
=
V
HYSO
A20 P
FDDD
VIP
x=
I
D-MAX
= I
LED
I
D-MAX
= (1 - D
MIN
) x I
LED
V
RD-MAX
= V
IN-MAX
+ V
O
V
RD-MAX
= V
O
V
RD-MAX
= V
IN-MAX
DSON
2
RMSTT
RIP
x=
-
I
RMST
=
-
D
x
I
LED
D
c
DI
T-
I
LEDRMS
x=
I
T-MAX
= x I
LED
1 - D
MAX
D
MAX
I
T-MAX
= D
MAX
x I
LED
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009 www.ti.com
The current rating should be at least 10% higher than the maximum average NFET current (I
T-MAX
):
Buck
(81)
Boost and Buck-boost
(82)
Approximate the nominal RMS transistor current (I
T-RMS
) :
Buck
(83)
Boost and Buck-boost
(84)
Given an NFET with on-resistance (R
DS-ON
), solve for the nominal power dissipation (P
T
):
(85)
12. DIODE
The Schottky diode voltage rating should be at least 15% higher than the maximum blocking voltage (V
RD-MAX
):
Buck
(86)
Boost
(87)
Buck-boost
(88)
The current rating should be at least 10% higher than the maximum average diode current (I
D-MAX
):
Buck
(89)
Boost and Buck-boost
(90)
Replace D
MAX
with D in the I
D-MAX
equation to solve for the average diode current (I
D
). Given a diode with forward
voltage (V
FD
), solve for the nominal power dissipation (P
D
):
(91)
13. OUTPUT OVLO
For boost and buck-boost regulators, output OVLO is programmed with the turn-off threshold voltage (V
TURN-OFF
)
and the desired hysteresis (V
HYSO
). To set V
HYSO
, solve for R
OV2
:
(92)
To set V
TURN-OFF
, solve for R
OV1
:
Boost
(93)
Buck-boost
(94)
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