LM3407_09 Datasheet, PDF (9/16 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

FIGURE 2. LM3407 Switching Waveforms
The switching frequency and duty ratio of the converter equal:
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By comparing the area of V_ISNS and V_RP over the ON period,
an error signal is generated. Such a comparison is function-
ally 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 3 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
3 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 induc-
tance 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 4 shows two sets of current sense sig-
nals named V_ISNS1 and V_ISNS2 that have identical frequencies
and duty cycles but different shapes of trapezoidal wave-
forms, each generating identical PWM signals.
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FIGURE 3. Pulse-Level Transformation
When V_MSL is higher than V_REF, the peak value of V_RP, the
switching duty cycle of the power switch will be reduced to
lower V_MSL. When V_MSL is lower than the peak value of V_RP,
the switching duty cycle of the power switch will be increased
to raise V_MSL. For example, when I_OUT is decreased, V_MSL will
become lower than V_REF. In order to maintain output current
regulation, the switching duty cycle of the power switch will
be increased and eventually push up V_MSL until V_MSL equals
V_REF. Since in typical floating buck regulators V_MSL is equal
to I_OUT * R_ISNS, true average output current regulation can be
achieved by regulating V_MSL. Figure 5 shows the waveforms
of V_ISNS and V_RP under closed loop operation.
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