LM3429MH National Semiconductor, LM3429MH Datasheet - Page 8

• 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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The average output LED current (I
average inductor current (I
trolled, I
input voltage or output voltage, the ideal duty cycle (D) is var-
ied to regulate I
D is a function of the conversion ratio:
Buck
Boost
Buck-boost
PREDICTIVE OFF-TIME (PRO) CONTROL
PRO control is used by the LM3429 to control I
combination of average peak current control and a one-shot
off-timer that varies with input voltage. The LM3429 uses
peak current control to regulate the average LED current
through an array of HBLEDs. This method of control uses a
series resistor in the LED path to sense LED current and can
use either a series resistor in the MosFET path or the MosFET
R
feed forward. D is indirectly controlled by changes in both
t
Even though the off-time control is quasi-hysteretic, the input
voltage proportionality in the off-timer creates an essentially
constant switching frequency over the entire operating range
for boost and buck-boost topologies. The buck topology can
be designed to give constant ripple over either input voltage
or output voltage, however switching frequency is only con-
stant at a specific operating point .
This type of control minimizes the control loop compensation
necessary in many switching regulators, simplifying the de-
sign process. The averaging mechanism in the peak detec-
tion control loop provides extremely accurate LED current
regulation over the entire operating range.
OFF
DS-ON
and t
for both cycle-by-cycle current limit and input voltage
LED
ON
, which vary depending on the operating point.
will be well regulated. As the system changes
L
and ultimately I
L
) , therefore if I
LED
. For any current regulator,
LED
FIGURE 1. Ideal CCM Regulator Inductor Current i
) is proportional to the
L
is tightly con-
LED
. It is a
8
PRO control was designed to mitigate “current mode
instability” (also called “sub-harmonic oscillation”) found in
standard peak current mode control when operating near or
above 50% duty cycles. When using standard peak current
mode control with a fixed switching frequency, this condition
is present, regardless of the topology. However, using a con-
stant off-time approach, current mode instability cannot oc-
cur, enabling easier design and control.
Predictive off-time advantages:
•
•
The only disadvantage is that synchronization to an external
reference frequency is generally not available.
SWITCHING FREQUENCY
An external resistor (R
the switch node (where D1, Q1, and L1 connect), in combi-
nation with a capacitor (C
sets the off-time (t
buck-boost topologies, the V
tually constant switching frequency (f
For a buck topology, R
however the V
switching frequency. Instead, constant ripple operation can
be achieved. Changing the connection of R
V
FIGURE 2. Off-timer Circuitry for Boost and Buck-boost
SW
There is no current mode instability at any duty cycle.
Higher duty cycles / voltage transformation ratios are
possible, especially in the boost regulator.
to V
IN
will provide a constant ripple over varying V
IN
proportionality will not ensure a constant
OFF
L
) as shown in
T
(t)
) connected between the RCT pin and
T
Regulators
T
) between the RCT and AGND pins,
and C
IN
proportionality ensures a vir-
T
are also used to set t
30094498
Figure
SW
).
T
2. For boost and
30094499
in
Figure 2
from
OFF
IN
,
.