LM3478

LM3478-Q1

www.ti.com

SNVS085V – JULY 2000 – REVISED FEBRUARY 2013

Figure 30. Shutdown Operation in Frequency Adjust Mode

SHORT-CIRCUIT PROTECTION

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 31. 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:

(8)

Including the voltage drop across the MOSFET and the diode the definition for the duty cycle is:

(9)

Copyright © 2000–2013, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM3478 LM3478-Q1