LM3423MHBKBSTEV/NOPB National Semiconductor, LM3423MHBKBSTEV/NOPB Datasheet - Page 4

• Manufacturer Part Number: LM3423MHBKBSTEV/NOPB
• Description: BOARD EVAL BUCK BOOST LM3423
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3423MHBKBSTEV/NOPB

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 35V
• Features: Dimmable
• Voltage - Input: 4.5 ~ 35V
• Utilized Ic / Part: LM3423
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM3423MHBKBSTEV

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This establishes a current gain determined by a resistor ratio
consisting of R17 and R7 along with R9 as described in the
equation:
Substituting in the resistor values as listed in the board
schematic gives a fixed I
SETTING THE CURRENT SENSE VOLTAGE
By substituting in different resistor values, the LED average
current can be user adjusted. The LM3423 controller uses a
high-side sense amplifier to regulate LED average current.
The CSH pin is regulated by the error amplifier to be V
Understanding how average LED current is regulated comes
down to understanding the relationship between V
V
side amplifier in forces its input terminals to equal potential.
Because of this, the V
other way to view this is that the amplifier’s output transistor
pulls current through R7 (R
happens when the voltage across R7 is equal to V
The current flowing down to the CSH pin is given by,
and the voltage at the CSH pin is then given by,
SNS
, because V
SNS
and R
SNS
LED
voltage is forced across R
SNS
HSP
current of 1A.
set the LED current. The high
) until V
HSP
FIGURE 2. Inductor Current Waveform
= V
HSN
SNS
CSH
HSP
and this
.
. An-
REF
and
.
4
The CSH voltage is the sense voltage gained up by the ratio
of R17 to R7. In addition, the control system’s error amplifier
regulates the CSH voltage to V
following equations are derived,
The above equations show how current in the LED relates to
the regulated voltage V
the LM3423.
The selection of resistors is not arbitrary, for matching and
noise performance, the CSH current should be set to be
around 100 µA. This current does not flow in the LEDs and
will not affect either off state LED current or the regulated LED
current. CSH current can be above or below this value, but
high side amplifier offset characteristics and jitter perfor-
mance may be affected slightly.
INDUCTOR SELECTION
The inductor should be chosen so that the current ripple
(Δi
through the inductor.
L
) is between 20% and 40% of the average current (<I
REF
, which is approximately 1.25V for
REF
. Using equation 14, the
30080823
L
>)