LM3481MM Datasheet, PDF (16/30 Pages) Texas Instruments – LM3481/LM3481Q High Efficiency Low-Side N-Channel Controller for Switching

LM3481
SNVS346E – NOVEMBER 2007 – REVISED APRIL 2012
www.ti.com
The important quantities in determining a proper inductance value are I_L (the average inductor current) and Δi_L
(the inductor current ripple difference between the peak inductor current and the average inductor current). If Δi_L
is larger than I_L, the inductor current will drop to zero for a portion of the cycle and the converter will operate in
discontinuous conduction mode. If Δi_L is smaller than I_L, the inductor current will stay above zero and the
converter will operate in continuous conduction mode. All the analysis in this datasheet assumes operation in
continuous conduction mode. To operate in continuous conduction mode, the following conditions must be met:
I_L > Δi_L
Choose the minimum I_OUT to determine the minimum L. A common choice is to set (2 x Δi_L) to 30% of I_L.
Choosing an appropriate core size for the inductor involves calculating the average and peak currents expected
through the inductor. In a boost converter,
I_L
=
I_OUT
1-D
I_{L_peak} = I_L(max) + Δi_L(max)
A core size with ratings higher than these values should be chosen. If the core is not properly rated, saturation
will dramatically reduce overall efficiency.
The LM3481 can be set to switch at very high frequencies. When the switching frequency is high, the converter
can operate with very small inductor values. With a small inductor value, the peak inductor current can be
extremely higher than the output currents, especially under light load conditions.
The LM3481 senses the peak current through the switch. The peak current through the switch is the same as the
peak current calculated above.
Programming the Output Voltage and Output Current
The output voltage can be programmed using a resistor divider between the output and the feedback pins, as
shown in Figure 31. The resistors are selected such that the voltage at the feedback pin is 1.275V. R_F1 and R_F2
can be selected using the equation,
V_OUT
=
1.275
(1+
R_{F1 )}
R_F2
A 100 pF capacitor may be connected between the feedback and ground pins to reduce noise.
The maximum amount of current that can be delivered at the output can be controlled by the sense resistor,
R_SEN. Current limit occurs when the voltage that is generated across the sense resistor equals the current sense
threshold voltage, V_SENSE. Limits for V_SENSE have been specified in the electrical characteristics section. This can
be expressed as:
I_sw(peak) x R_SEN = V_SENSE- D x V_SL
The peak current through the switch is equal to the peak inductor current.
I_sw(peak) = I_L(max) + Δi_L
Therefore for a boost converter
I_OUT(max) (D x V_IN)
I_sw(peak) =
(1-D)
+
(2 x f_S x L)
16
Submit Documentation Feedback
Product Folder Links: LM3481
Copyright © 2007–2012, Texas Instruments Incorporated