LM3424 Datasheet, PDF (17/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Feature Description (continued)
There are two possible methods to sense the transistor current. The R_DS-ON of the main power MOSFET can be
used as 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 located 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_LIM) can be calculated using either method as the limiting resistance (R_LIM):
I_LIM
=
245 mV
R_LIM
(12)
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 together local to the controller and a single
connection is made to GND.
7.3.6 Slope Compensation
The LM3424 has programmable slope compensation in order to provide stability over a wide range of operating
conditions. Without slope compensation, a well-known condition called current mode instability (or sub-harmonic
oscillation) can result if there is a perturbation of the MOSFET current sense voltage at the IS pin, due to noise
or a some type of transient.
Through a mathematical / geometrical analysis of the inductor current (I_L) and the corresponding control current
(I_C, it can be shown that if D < 0.5, the effect of the perturbation will decrease each switching cycle and the
system will remain stable. However, if D > 0.5 then the perturbation will grow as shown in Figure 23, eventually
causing a "period doubling" effect where the effect of the perturbation remains, yielding current mode instability.
Looking at Figure 22, the positive PWM comparator input is the IS voltage, a mirror of I_L during t_ON, plus a typical
900 mV offset. The negative input of the PWM comparator is the COMP pin which is proportional to I_C, the
threshold at which the main MOSFET (Q1) is turned off.
The LM3424 mitigates current mode instability by implementing an aritifical ramp (commonly called slope
compensation) which is summed with the sensed MOSFET current at the IS pin as shown in Figure 22. This
combined signal is compared to the COMP pin to generate the PWM signal. An increase in the ramp that is
added to the sense voltage will increase the maximum achievable duty cycle. It should be noted that as the
artificial ramp is increased more and more, the control method approaches standard voltage mode control and
the benefits of current mode control are reduced.
To program the slope compensation, an external resistor, R_SLP, is connected from SLOPE to GND. This sets the
slope of the artificial ramp that is added to the MOSFET current sense voltage. A smaller R_SLP value will increase
the slope of the added ramp. A simple calculation is suggested to ensure any duty cycle is attainable while
preventing the addition of excessive ramp. This method requires the artifical ramp slope (M_A) to be equal to half
the inductor slope during t_OFF:
M_{A = RT}
7.5 x 10¹²
x R _SLP x R_LIM
= ^VO
2 x L1
(13)
i_L (t)
I_C
Ideal
i_L (t)
Actual
i_L (t)
T_S
2T_S
0
t
Figure 23. "Period Doubling" due to Current Mode Instability
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