National Semiconductor LM3424, Руководство пользователя

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The current rating should be at least 10% higher than the
maximum average NFET current (I
T-MAX
):
Buck
Boost and Buck-boost
Approximate the nominal RMS transistor current (I
T-RMS
) :
Buck
Boost and Buck-boost
Given an NFET with on-resistance (R
DS-ON
), solve for the
nominal power dissipation (P
T
):
12. DIODE
The Schottky diode voltage rating should be at least 15%
higher than the maximum blocking voltage (V
RD-MAX
):
Buck
Boost
Buck-boost
The current rating should be at least 10% higher than the
maximum average diode current (I
D-MAX
):
Buck
Boost and Buck-boost
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
):
13. OUTPUT OVLO
For boost and buck-boost regulators, output OVLO is pro-
grammed with the turn-off threshold voltage (V
TURN-OFF
) and
the desired hysteresis (V
HYSO
). To set V
HYSO
, solve for R
OV2
:
To set V
TURN-OFF
, solve for R
OV1
:
Boost
Buck-boost
A small filter capacitor (C
OVP
= 47 nF) should be added from
the OVP pin to ground to reduce coupled switching noise.
14. INPUT UVLO
For all topologies, input UVLO is programmed with the turn-
on threshold voltage (V
TURN-ON
) and the desired hysteresis
(V
HYS
).
Method #1: If no PWM dimming is required, a two resistor
network can be used. To set V
HYS
, solve for R
UV2
:
To set V
TURN-ON
, solve for R
UV1
:
Method #2: If PWM dimming is required, a three resistor net-
work is suggested. To set V
TURN-ON
, assume R
UV2
= 10 k
Ω
and solve for R
UV1
as in Method #1. To set V
HYS
, solve for
R
UVH
:
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LM3424