Datasheet

f
SW
=
t
ON
+ t
OFF
1
D =
t
ON
+ t
OFF
t
ON
and
V
RP
I
LX
V
ISNS
V
MSL
time
time
time
time
t
OFF
t
ON
T
I
L1
I
OUT
V
REF
I
OUT
= I
L1(AVG)
LM3407
SNVS553B –JANUARY 2008–REVISED MAY 2013 www.ti.com
Figure 20. LM3407 Switching Waveforms
The switching frequency and duty ratio of the converter equal:
(1)
By comparing the area of V
ISNS
and V
RP
over the ON period, an error signal is generated. Such a comparison is
functionally equivalent to comparing the middle level of I
SNS
to V
RP
during the ON-period of a switching cycle.
The error signal is fed to a PWM comparator circuit to produce the PWM control pulse to drive the internal power
N-MOSFET. Figure 21 shows the implementation of the PWM switching signal. The error signal is fed to a PWM
comparator circuit to produce the PWM control pulse to drive the internal power N-MOSFET. Figure 21 shows
the implementation of the PWM switching signal.
In closed loop operation, the difference between V
MSL
and V
RP
is reflected in the changes of the switching duty
cycle of the power switch. This behavior is independent of the inductance of the inductor and input voltage
because for the same set of I
OUT
* R
ISNS
, ON time, and switching period, there exists only one V
MSL
. Figure 22
shows two sets of current sense signals named V
ISNS1
and V
ISNS2
that have identical frequencies and duty cycles
but different shapes of trapezoidal waveforms, each generating identical PWM signals.
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