LM3478MA_15 Datasheet, PDF (17/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
www.ti.com
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
V_SENSE - (D x V_SENSE x V_SLratio)
R_SEN =
ISW_LIMIT
(19)
R_SEN is part of the current mode control loop and has some influence on control loop stability. Therefore, once
the current limit threshold is set, loop stability must be verified. As described in the slope compensation section,
the following must hold true for a current mode converter to be stable:
S_f - S_e < S_n + S_e
(20)
To verify that this equation holds true, use the following equation:
R_SEN
<
2 x V_SL x f_S x L
Vo - (2 x V_IN)
(21)
If the selected RSEN is greater than this value, additional slope compensation must be added to ensure stability,
as described in the section below.
CURRENT LIMIT WITH EXTERNAL SLOPE COMPENSATION
R_SL is used to add additional slope compensation when required. It is not necessary in most designs and R_SL
should be no larger than necessary. Select RSL according to the following equation:
R_SL
>
R_SEN x (Vo - 2V_IN)
2 x f_S x L
40 PA
-
V_SL
(22)
Where R_SEN is the selected value based on current limit. With RSL installed, the control signal includes additional
external slope to stabilize the loop, which will also have an effect on the current limit threshold. Therefore, the
current limit threshold must be re-verified, as illustrated in the equations below :
V_CS = V_SENSE – (D x (V_SL + ΔV_SL))
(23)
Where ΔV_SL is the additional slope compensation generated as discussed in the slope compensation ramp
section and calculated as:
ΔV_SL = 40 µA x R_SL
(24)
This changes the equation for current limit (or R_SEN) to:
V_SENSE - (D x(V_SL + 'V_SL))
ISW_LIMIT =
R_SEN
(25)
The R_SEN and R_SL values may have to be calculated iteratively in order to achieve both the desired current limit
and stable operation. In some designs R_SL can also help to filter noise on the I_SEN pin.
If the inductor is selected such that ripple current is the recommended 30% value, and the current limit threshold
is 120% of the maximum peak, a simpler method can be used to determine R_SEN. The equation below will
provide optimum stability without RSL, provided that the above 2 conditions are met:
R_SEN =
V_SENSE
Vo - Vi
ISW_LIMIT + _{L x fS}
xD
(26)
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 its peak current. The peak diode current can be calculated using the formula:
I_D(Peak) = I_OUT/ (1−D) + ΔI_L
(27)
Thermally the diode must be able to handle the maximum average current delivered to the output. The peak
reverse voltage for boost converters is equal to the regulated output voltage. The diode must be capable of
handling this voltage. To improve efficiency, a low forward drop schottky diode is recommended.
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