LM3481MM Datasheet, PDF (17/30 Pages) Texas Instruments – LM3481/LM3481Q High Efficiency Low-Side N-Channel Controller for Switching

LM3481
www.ti.com
SNVS346E – NOVEMBER 2007 – REVISED APRIL 2012
Combining the two equations yields an expression for R_SEN
R_SEN =
V_SENSE - (D x V_SL)
I_OUT(max) (D x V_IN)
+
(1-D) (2 x f_S x L)
Evaluate R_SEN at the maximum and minimum V_IN values and choose the smallest R_SEN calculated.
V_IN
DR
LM3481
I_SEN
FB
L
D1
Q
R_SEN
V_OUT
+
C_OUT
R_F1
R_F2
Figure 31. Adjusting the Output Voltage
Current Limit with Additional Slope Compensation
If an external slope compensation resistor is used (see Figure 22) the internal control signal will be modified and
this will have an effect on the current limit.
If R_SL is used, then this will add to the existing slope compensation. The command voltage, V_CS, will then be
given by:
V_CS = V_SENSE − D x (V_SL + ΔV_SL)
Where V_SENSE is a defined parameter in the electrical characteristics section and ΔV_SL is the additional slope
compensation generated as discussed in the Slope Compensation Ramp section. This changes the equation for
R_SEN to:
R_SEN =
V_SENSE - D
I_OUT(max)
x
(V_SL+'V_SL)
(D x V_IN)
+
(1-D) (2 x f_S x L)
Note that since ΔV_SL = R_SL x K as defined earlier, R_SLcan be used to provide an additional method for setting the
current limit. In some designs R_SL can also be used to help filter noise to keep the I_SEN pin quiet.
Power Diode Selection
Observation of the boost converter circuit shows that the average current through the diode is the average load
current, and the peak current through the diode is the peak current through the inductor. The diode should be
rated to handle more than the inductor peak current. The peak diode current can be calculated using the formula:
I_D(Peak) = [I_OUT/ (1−D)] + Δi_L
In the above equation, I_OUT is the output current and Δi_L has been defined in Figure 30.
The peak reverse voltage for a boost converter is equal to the regulator output voltage. The diode must be
capable of handling this peak reverse voltage. To improve efficiency, a low forward drop Schottky diode is
recommended.
Copyright © 2007–2012, Texas Instruments Incorporated
Product Folder Links: LM3481
Submit Documentation Feedback
17