LM3478MA_15 Datasheet, PDF (13/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

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LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
Figure 29. Shutdown Operation in Frequency Adjust Mode
SHORT-CIRCUIT PROTECTION
When the voltage across the sense resistor measured on the I_SEN pin exceeds 343 mV, short circuit current limit
protection gets activated. A comparator inside the LM3478 reduces the switching frequency by a factor of 5 and
maintains this condition until the short is removed. In normal operation the sensed current will trigger the power
MOSFET to turn off. During the blanking interval the PWM comparator will not react to an over current so that
this additional 343 mV current limit threshold is implemented to protect the device in a short circuit or severe
overload condition.
Typical Applications
The LM3478 may be operated in either the continuous (CCM) or the discontinuous current conduction mode
(DCM). The following applications are designed for the CCM operation. This mode of operation has higher
efficiency and usually lower EMI characteristics than the DCM.
BOOST CONVERTER
The boost converter converts a low input voltage into a higher output voltage. The basic configuration for a boost
converter is shown in Figure 30. In the CCM (when the inductor current never reaches zero at steady state), the
boost regulator operates in two states. In the first state of operation, MOSFET Q is turned on and energy is
stored in the inductor. During this state, diode D is reverse biased and load current is supplied by the output
capacitor, Cout.
In the second state, MOSFET Q is off and the diode is forward biased. The energy stored in the inductor is
transferred to the load and the output capacitor. The ratio of the switch on time to the total period is the duty
cycle D:
D = 1 - (V_in / V_out)
(8)
Including the voltage drop across the MOSFET and the diode the definition for the duty cycle is:
D = 1 - ((V_in - V_q)/(V_out + V_d))
(9)
V_d is the forward voltage drop of the diode and V_q is the voltage drop across the MOSFET when it is on.
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