LTC3720_15 Datasheet, PDF (20/24 Pages) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

LTC3720
APPLICATIO S I FOR ATIO
By positioning the output voltage 60mV above the regula-
tion point at no load, it will only drop 65mV below the
regulation point after the load step, well within the ±100mV
tolerance.
Implementing active voltage positioning requires setting a
precise gain between the sensed current and the output
voltage. Because of the variability of MOSFET on-resis-
tance, it is prudent to use a sense resistor with active
voltage positioning. In order to minimize power lost in this
resistor, a low value is chosen of 0.003Ω. The nominal
sense voltage will now be:
V_SNS(NOM) = (0.003Ω)(15A) = 45mV
To maintain a reasonable current limit, the voltage on the
V_RNG pin is reduced to its minimum value of 0.5V, corre-
sponding to a 50mV nominal sense voltage.
Next, the gain of the LTC3720 error amplifier must be
determined. The change in I_TH voltage for a corresponding
change in the output current is:
∆I_TH
=


12V
V_RNG


R_SENSE
∆I_OUT
( )( )( ) ^{= 24 0.003Ω 15A = 1.08V}
The corresponding change in the output voltage is deter-
mined by the gain of the error amplifier and feedback
divider. The LTC3720 error amplifier has a transconduc-
tance g_m that is constant over both temperature and a wide
± 40mV input range. Thus, by connecting a load resis-
tance R_VP to the I_TH pin, the error amplifier gain can be
precisely set for accurate active voltage positioning.
∆I_TH
=

g_m R_{VP }
0.8V
V_OUT


∆V_OUT
Solving for this resistance value:
R_VP
=
V_OUT ∆I_TH
(0.8V)g_m ∆V_OUT
= ^{(1.5V)(1.08V)} = 9.53k
(0.8V)(1.7mS)(125mV)
The gain setting resistance R_VP is implemented with two
resistors, R_VP1 connected from I_TH to ground and R_VP2
connected from I_TH to INTV_CC. The parallel combination of
these resistors must equal R_VP and their ratio determines
nominal value of the I_TH pin voltage when the error
amplifier input is zero. To center the load line around the
regulation point, the I_TH pin voltage must be set to corre-
spond to half the output current. The relation between I_TH
voltage and the output current is:
I_TH(NOM)
=


12V
V_RNG


^RSENSE^IOUT
–
1
2
∆I_L


+
0.8V
( ) ⁼


12V
0.5V


0.003Ω

^{7.5A –}
1
2

4.7A_
+ 0.8V
= 1.17V
Solving for the required values of the resistors:
R_VP1
=
5V
5V
– I_TH(NOM)
R_VP
=
5V
5V
– 1.17V
9.53k
= 12.44k
R_VP2
=
5V
I_TH(NOM)
R_VP
=
5V _9.53k
1.17V
=
40.73k
3720f
20