LM2747_15 Datasheet, PDF (18/38 Pages) Texas Instruments – Synchronous Buck Controller with Pre-Bias Startup, and Optional Clock Synchronization

LM2747
SNVS370B – MARCH 2006 – REVISED MARCH 2013
Normal Operation
ILIM
Current Limit
www.ti.com
IL
D
Figure 29. Current Limit Threshold
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 typical 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:
V_IN - V_O
I_PK-CL = I_LIM + (T_SW - 200 ns)
L
Where
• T_SW is the inverse of switching frequency f_SW.
(9)
The 200 ns 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 discharges the soft-start capacitor through a fixed 90 µA sink. The output of the LM2747 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 discharge is a
smooth, controlled ramp of output voltage when the current limit condition is cleared.
SHUTDOWN
If the shutdown pin is pulled low, (below 0.8V) the LM2747 enters shutdown mode, and discharges the soft-start
capacitor through a MOSFET switch. The high and low-side MOSFETs are turned off. The LM2747 remains in
this state as long as V_SD sees a logic low (see the ELECTRICAL CHARACTERISTICS 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_CC or to another voltage between 1.3V to V_CC (see the ELECTRICAL CHARACTERISTICS 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_IN) to 1.2V (V_OUT) at a maximum load of 4A with an
efficiency of 89% and a switching frequency of 300 kHz. The same procedures can be followed to create many
other designs with varying input voltages, output voltages, and load currents.
18
Submit Documentation Feedback
Product Folder Links: LM2747
Copyright © 2006–2013, Texas Instruments Incorporated