LM3430_07 NSC [National Semiconductor], LM3430_07 Datasheet - Page 14

• Manufacturer Part Number: LM3430_07
• Description: LED Backlighting Solution with LM3430 and LM3432
• Manufacturer: NSC [National Semiconductor]

www.national.com
Selecting the Current Sensing Resistor
The current sensing resistor, R
regulation of the inductor current and to provide cycle by cycle
current limit function. The resistance selected must be low
enough to keep the power dissipation to a minimum and still
can maintain good signal-to-noise ratio for the current sensing
circuitry. The current limit comparator's threshold is 0.5V. The
resistance should be selected so that the switching cycle can
be terminated before the inductor current exceeds the satu-
ration rating of the inductor. The required resistor calculation
must take into account of both the switch current through the
sensing resistor and the compensation ramp current flowing
through the internal 2 kΩ resistor and external current sensing
network resistors. The worst case average power dissipation
in the current sensing resistor, P
equation in below.
Where the D
a minimum.
The Control Loop Compensation
The control loop is comprised of two parts. The first part is the
power stage, which consists of the pulse width modulator, the
output filter and the load. The second part is the error amplifier
which is realized by an op-amp configured as an inverting
amplifier. To close the control loop, compensation is required
to ensure stability and optimize system performance. Many
techniques exist for selecting the compensation network com-
ponents. The most popular method is to create the Bode plots
of gain and phase for the power stage and error amplifier in-
dividually. By combining both stages, the open loop system
Bode plots resulted. By using the plots, overall bandwidth,
gain margin and phase margin of the regulator can be easily
determined. Software tools such as MathCAD, Matlab and
Excel can be used to observe how the changes in compen-
sation network and power stage affecting the system gain and
phase. One approach to select the compensation network is
introduced in the LM3430 datasheet in details. The theoreti-
cally calculated compensation network can only be used as
the starting point and bench testing and fine tuning is required
to come up with the final values. With the demonstration
board, a type II compensation network is suggested and the
respective component values are listed in below:
Power Stage:
L = 22 µH
R
C
R
R
F
Output Voltage Feedback Divider:
R13 = 118 kΩ
R9 = 3.01 kΩ
Compensation Network:
R12 = 118 kΩ
C4 = 47 nF
C9 = 12 pF
SW
OUT
OUT
ESR
SNS
= 1 MHz
= 417Ω
= 22 µF
= 350mΩ
= 0.2Ω
MAX
is the On Duty Ratio with the input voltage is
SNS
SNS
is used for steady state
can be estimated by the
14
CURRENT REGULATOR DESIGN WITH LM3432
The LM3432 provides a simple and handy solution to drive
strings of serially connected LEDs with precisely controlled
constant current. Only couple of external passive compo-
nents, up to about 120 LEDs in six strings can be lighted up.
To control the brightness of the LED strings, both analog and
digital dimming method can be used.
Programming the LED Current
The string current can be programmed by an external resistor,
R
calculate the resistance is:
With this demonstration board, the string current is 20 mA and
the resistor is R25. Applying to the equation:
Determination of the Analog Dimming Frequency
In analog dimming mode, the internally generated PWM fre-
quency is controlled by the external capacitor, C
nected across MODE pin and GND. The equation that
governs the relationship is:
Where C
In this demonstration board, the capacitor to determine the
PWM frequency is C34 and the capacitance used is 680 pF.
The PWM frequency is:
IREF
connected across IREF pin and GND. The equation to
MODE
is in Farads and F
Power Stage Amplifier
PWM
is in Hz.
MODE
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con-