LM3424BKBSTEVAL National Semiconductor, LM3424BKBSTEVAL Datasheet - Page 12

• Manufacturer Part Number: LM3424BKBSTEVAL
• Description: BOARD, EVALUATION, FOR LM3424BKBST
• Manufacturer: National Semiconductor

Specifications of LM3424BKBSTEVAL

• Kit Contents: Assembled LM3424 Buck-Boost Evaluation Board, Application Note, LM3424 Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: 1A Output, Thermal Foldback, Analogue Dimming, High Frequency
• Mcu Supported Families: LM3424

www.national.com
AVERAGE LED CURRENT
To first understand how the LM3424 regulates LED current,
the thermal foldback functionality will be ignored.
shows the physical implementation of the LED current sense
circuitry assuming the thermal foldback circuitry is a simple
current source which, for now, will be set to zero (I
The LM3424 uses an external current sense resistor (R
placed in series with the LED load to convert the LED current
(I
inputs to the high-side sense amplifier which are forced to be
equal potential (V
cause of this, the V
generates a current that is summed with the thermal foldback
current (I
out of the CSH pin and through the R
amplifier will regulate the CSH pin to 1.24V and assuming
I
This means V
TF
LED
= 0A, I
) into a voltage (V
TF
CSH
) to generate the signal current (I
SNS
can be calculated:
HSP
will be regulated as follows:
SNS
=V
SNS
HSN
voltage is forced across R
). The HSP and HSN pins are the
) through negative feedback. Be-
CSH
resistor. The error
FIGURE 3. LED Current Sense Circuitry
CSH
) which flows
FIGURE 2. Timing Circuitry
HSP
TF
Figure 3
= 0A).
which
SNS
)
12
I
The selection of the three resistors (R
not arbitrary. For matching and noise performance, the sug-
gested signal current I
current does not flow in the LEDs and will not affect either the
off-state LED current or the regulated LED current. I
be above or below this value, but the high-side amplifier offset
characteristics may be affected slightly. In addition, to mini-
mize the effect of the high-side amplifier voltage offset on LED
current accuracy, the minimum V
50 mV. Finally, a resistor (R
series with the HSN pin to cancel out the effects of the input
bias current (~10 µA) of both inputs of the high-side sense
amplifier.
Note that he CSH pin can also be used as a low-side current
sense input regulated to 1.24V. The high-side sense amplifier
is disabled if HSP and HSN are tied to GND.
LED
can then be calculated:
30085799
CSH
HSN
is approximately 100 µA. This
= R
SNS
HSP
30085757
SNS
) should be placed in
is suggested to be
, R
CSH
, and R
CSH
HSP
can
) is