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 k

Ω 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 k

Ω

R9 = 3.01 k

Ω

Compensation Network:

R12 = 118 k

Ω

C4 = 47 nF

C9 = 12 pF

30028915

Power Stage Amplifier

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:

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