lm2746mxax National Semiconductor Corporation, lm2746mxax Datasheet - Page 14

• Manufacturer Part Number: lm2746mxax
• Description: Low Voltage N-channel Mosfet Synchronous Buck Regulator Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
as inductor current has fallen to the current limit threshold.
The LM2746 will continue to skip high-side MOSFET pulses
until the inductor current peak is below the current limit
threshold, at which point the system resumes normal opera-
tion.
Unlike a high-side MOSFET current sensing scheme, which
limits the peaks of inductor current, low-side current sensing
is only allowed to limit the current during the converter
off-time, when inductor current is falling. Therefore in a typi-
cal current limit plot the valleys are normally well defined, but
the peaks are variable, according to the duty cycle. The
PWM error amplifier and comparator control the off-pulse of
the high-side MOSFET, even during current limit mode,
meaning that peak inductor current can exceed the current
limit threshold. Assuming that the output inductor does not
saturate, the maximum peak inductor current during current
limit mode can be calculated with the following equation:
Where T
200ns term represents the minimum off-time of the duty
cycle, which ensures enough time for correct operation of
the current sensing circuitry.
In order to minimize the time period in which peak inductor
current exceeds the current limit threshold, the IC also dis-
charges the soft-start capacitor through a fixed 90 µA sink.
The output of the LM2746 internal error amplifier is limited by
the voltage on the soft-start capacitor. Hence, discharging
the soft-start capacitor reduces the maximum duty cycle D of
the controller. During severe current limit this reduction in
duty cycle will reduce the output voltage if the current limit
conditions last for an extended time. Output inductor current
will be reduced in turn to a flat level equal to the current limit
threshold. The third benefit of the soft-start capacitor dis-
charge is a smooth, controlled ramp of output voltage when
the current limit condition is cleared.
SW
FIGURE 10. Current Limit Threshold
is the inverse of switching frequency f
(Continued)
SW
20147788
. The
14
SHUTDOWN
If the shutdown pin is pulled low, (below 0.8V) the LM2746
enters shutdown mode, and discharges the soft-start capaci-
tor through a MOSFET switch. The high and low-side MOS-
FETs are turned off. The LM2746 remains in this state as
long as V
istics table). To assure proper IC start-up the shutdown pin
should not be left floating. For normal operation this pin
should be connected directly to V
between 1.3V to V
table).
DESIGN CONSIDERATIONS
The following is a design procedure for all the components
needed to create the Typical Application Circuit shown on
the front page. This design converts 3.3V (V
(V
and a switching frequency of 300kHz. The same procedures
can be followed to create many other designs with varying
input voltages, output voltages, and load currents.
Duty Cycle Calculation
The complete duty cycle for a buck converter is defined with
the following equation:
where V
drops that develop across the low side and high side MOS-
FETs. Assuming the inductor ripple current is 20% to 30% of
the output current, therefore:
To calculate the maximum duty cycle use the estimated ’hot’
R
and maximum load. As shown in Figure 11, the worst case
maximum duty cycles of the LM2746 occurs at 125˚C junc-
tion temperature vs V
that the operating duty cycle is below the curve in Figure 11,
if this condition is not satisfied, the system will be unable to
develop the required duty cycle to derive the necessary
system power and so the output voltage will fall out of
regulation.
DS(on)
OUT
) at a maximum load of 4A with an efficiency of 89%
value of the MOSFETs, the minimum input voltage,
SWL
SD
sees a logic low (see the Electrical Character-
and V
V
V
SWH
SWL
(High-Side MOSFET)
(Low-Side MOSFET)
SWH
CC
CC
= I
= I
(see the Electrical Characteristics
are the respective forward voltage
(IC control section voltage). Ensure
OUT
OUT
x R
x R
DS(on)LOW
DS(on)HIGH
CC
or to another voltage
IN
) to 1.2V