Datasheet

I
LIM
=
245 mV
R
LIM
LM3424
I
T
PWM
COMP
IS
R
LIM
Q1
GATE
LEB
GND
0.245V
0.9V
Ramp
Ramp
Ramp
Generator
SLOPE
R
DS-ON
Sensing
R
LIM
Sensing
R
SLP
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009 www.ti.com
THERMAL SHUTDOWN
The LM3424 includes thermal shutdown. If the die temperature reaches approximately 165°C the device will shut
down (GATE pin low), until it reaches approximately 140°C where it turns on again.
CURRENT SENSE/CURRENT LIMIT
The LM3424 achieves peak current mode control using a comparator that monitors the main MosFET (Q1)
transistor current, comparing it with the COMP pin voltage as shown in Figure 25. Further, it incorporates a
cycle-by-cycle over-current protection 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 internal N-channel MosFET which pulls it down at the
conclusion of every cycle. The discharge device remains on an additional 240 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
ON-MIN
).
Figure 25. Current Sense / Current Limit Circuitry
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
):
(13)
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.
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 26, eventually
causing a "period doubling" effect where the effect of the perturbation remains, yielding current mode instability.
Looking at Figure 25, 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.
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