LM3407 Datasheet, PDF (10/22 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

LM3407
SNVS553B – JANUARY 2008 – REVISED MAY 2013
I_L1
I_OUT
I_OUT = I_L1(AVG)
I_LX
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time
V_ISNS
time
V_MSL
V_RP
V_REF
time
time
t_ON t_OFF
T
Figure 20. LM3407 Switching Waveforms
The switching frequency and duty ratio of the converter equal:
D
=
t_ON
t_ON + t_OFF
and
1
f_SW = _{tON + tOFF}
(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.
10
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