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| LM3407 Datasheet, PDF (11/22 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs | |||
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www.ti.com
VISNS1
LM3407
SNVS553B â JANUARY 2008 â REVISED MAY 2013
VMSL
0
VPWM
0
VISNS2
VMSL
0
Figure 21. Pulse-Level Transformation
When VMSL is higher than VREF, the peak value of VRP, the switching duty cycle of the power switch will be
reduced to lower VMSL. When VMSL is lower than the peak value of VRP, the switching duty cycle of the power
switch will be increased to raise VMSL. For example, when IOUT is decreased, VMSL will become lower than VREF.
In order to maintain output current regulation, the switching duty cycle of the power switch will be increased and
eventually push up VMSL until VMSL equals VREF. Since in typical floating buck regulators VMSL is equal to IOUT *
RISNS, true average output current regulation can be achieved by regulating VMSL. Figure 23 shows the
waveforms of VISNS and VRP under closed loop operation.
VRP
D/fSW
VREF
0
D/fSW
VPWM
1/fSW
1/fSW
+
0
Error
-Amplifier
PWM signal
Generator
To power switch
VISNS
VMSL
0
D/fSW
1/fSW
PWM
sawtooth
T
Figure 22. Implementation of the PWM Switching Signal
Copyright © 2008â2013, Texas Instruments Incorporated
Product Folder Links: LM3407
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