LM3478_09 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

The FA/SD pin also functions as a shutdown pin. If a high
signal (>1.35V) appears on the FA/SD pin, the LM3478 stops
switching and goes into a low current mode. The total supply
current of the IC reduces to less than 10 uA under these con-
ditions. Figure 7 shows implementation of the shutdown func-
tion when operating in frequency adjust mode. In this mode a
high signal for more than 30us shuts down the IC. However,
the voltage on the FA/SD pin should be always less than the
absolute maximum of 7V to avoid any damage to the device.
FIGURE 6. Frequency Adjust
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FIGURE 7. Shutdown Operation in Frequency Adjust Mode
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SHORT-CIRCUIT PROTECTION
When the voltage across the sense resistor measured on the
Isen 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 con-
dition 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 re-
act 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 low-
er 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 8. In the CCM (when the inductor current
never reaches zero at steady state), the boost regulator op-
erates 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)
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))
V_d is the forward voltage drop of the diode and V_q is the volt-
age drop across the MOSFET when it is on.
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