LM3429MH National Semiconductor, LM3429MH Datasheet - Page 10

• Manufacturer Part Number: LM3429MH
• Description: IC, LED DRIVER, CNTRL, 14TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3429MH

• Led Driver Application: Automotive, General Illumination
• No. Of Outputs: 1
• Output Current: 1A
• Input Voltage: 4.5V To 75V
• Dimming Control Type: Analogue / PWM
• Topology: Boost, Buck, Buck-Boost, SEPIC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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ANALOG DIMMING
The CSH pin can be used to analog dim the LED current by
adjusting the current sense voltage (V
different methods to adjust V
1.
2.
In general, analog dimming applications require a lower
switching frequency to minimize the effect of the leading edge
blanking circuit. As the LED current is reduced, the output
voltage and the duty cycle decreases. Eventually, the mini-
mum on-time is reached. The lower the switching frequency,
the wider the linear dimming range.
methods are physically implemented.
Method 1 uses an external potentiometer in the CSH path
which is a simple addition to the existing circuitry. However,
the LEDs cannot dim completely because there is always
some resistance causing signal current to flow. This method
is also susceptible to noise coupling at the CSH pin since the
potentiometer increases the size of the signal current loop.
Method 2 provides a complete dimming range and better
noise performance, though it is more complex. It consists of
a PNP current mirror and a bias network consisting of an NPN,
2 resistors and a potentiometer (R
the amount of current sourced into the CSH pin. A higher re-
sistance value will source more current into the CSH pin
causing less regulated signal current through R
ly dimming the LEDs. V
voltage reference, while Q7 and Q8 should be a dual pair PNP
for best matching and performance. The additional current
(I
The corresponding I
ADD
External variable resistance : Adjust a potentiometer
placed in series with R
External variable current source: Source current (0 µA to
I
) sourced into the CSH pin can be calculated:
CSH
) into the CSH pin to adjust V
FIGURE 5. Analog Dimming Circuitry
LED
for a specific I
REF
CSH
SNS
should be a precise external
to vary V
using the CSH pin:
ADJ
Figure 5
SNS
SNS
), where R
ADD
SNS
). There are several
.
is:
.
shows how both
HSP
ADJ
, effective-
controls
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10
CURRENT SENSE/CURRENT LIMIT
The LM3429 achieves peak current mode control using a
comparator that monitors the MosFET transistor current,
comparing it with the COMP pin voltage as shown in
6. Further, it incorporates a cycle-by-cycle over-current pro-
tection function. Current limit is accomplished by a redundant
internal current sense comparator. If the voltage at the current
sense comparator input (IS) exceeds 245 mV (typical), the on
cycle is immediately terminated. The IS input pin has an in-
ternal N-channel MosFET which pulls it down at the conclu-
sion of every cycle. The discharge device remains on an
additional 250 ns (typical) after the beginning of a new cycle
to blank the leading edge spike on the current sense signal.
The leading edge blanking (LEB) determines the minimum
achievable on-time (t
There are two possible methods to sense the transistor cur-
rent. The R
the current sense resistance because the IS pin was designed
to withstand the high voltages present on the drain when the
MosFET is in the off state. Alternatively, a sense resistor lo-
cated in the source of the MosFET may be used for current
sensing, however a low inductance (ESL) type is suggested.
The cycle-by-cycle current limit (I
either method as the limiting resistance (R
In general, the external series resistor allows for more design
flexibility, however it is important to ensure all of the noise
sensitive low power ground connections are connected to-
gether local to the controller and a single connection is made
to the high current PGND (sense resistor ground point).
FIGURE 6. Current Sense / Current Limit Circuitry
DS-ON
of the main power MosFET can be used as
ON-MIN
).
LIM
) can be calulated using
LIM
):
Figure
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