MAX5038A Datasheet, PDF (16/26 Pages) Maxim Integrated Products – Dual-Phase, Parallelable, Average-Current-Mode Controllers

Dual-Phase, Parallelable, Average-Current-Mode
Controllers
Differential Amplifier
The differential amplifier (DIFF AMP) facilitates output
voltage remote sensing at the load (Figures 3a and 3b).
It provides true differential output voltage sensing while
rejecting the common-mode voltage errors due to high-
current ground paths. Sensing the output voltage
directly at the load provides accurate load voltage
sensing in high-current environments. The VEA pro-
vides the difference between the differential amplifier
output (DIFF) and the desired output voltage. The dif-
ferential amplifier has a bandwidth of 3MHz. The differ-
ence between SENSE+ and SENSE- regulates to the
preset output voltage for the MAX5038A and regulates
to +1V for the MAX5041A.
Voltage-Error Amplifier
The VEA sets the gain of the voltage control loop and
determines the error between the differential amplifier
output and the internal reference voltage (V_REF).
V_REF equals V_OUT(NOM) for the +1.8V or lower voltage
versions of the MAX5038A and V_REF equals V_OUT(NOM)/2
for the +2.5V and +3.3V versions. For MAX5041A, V_REF
equals +1V.
An offset is added to the output voltage of the
MAX5038A/MAX5041A with a finite gain (R_F/R_IN) of the
VEA such that the no-load output voltage is higher than
the nominal value. Choose R_F and R_IN from the
Adaptive Voltage Positioning section and use the follow-
ing equations to calculate the no-load output voltage.
MAX5038A:
V_OUT(NL)
=

¹⁺
R_IN


×
V_OUT(NOM)
(3)
 R_F 
MAX5041A:
V_OUT(NL)
=

¹⁺
R_IN


×


R_H
+ R_L


×
V_REF
(4)
 R_F   R_L 
where R_H and R_L are the feedback resistor network
(Figure 2).
Some applications require V_OUT equal to V_OUT(NOM) at
no load. To ensure that the output voltage does not
exceed the nominal output voltage (V_OUT(NOM)), add a
resistor R_X from V_CC to EAN.
Use the following equations to calculate the value of R_X.
For MAX5038A versions of V_OUT(NOM) ≤ +1.8V:
R_X = [V_CC − (V_NOM + 0.6)] × ^RF
(5)
V_NOM
For MAX5038A versions of V_OUT(NOM) > +1.8V:
R_X = [2V_CC − (V_NOM + 1.2)] × ^RF
(6)
V_NOM
For MAX5041A:
R_X = [V_CC − 1.6] × ^RF
(7)
V_REF
The VEA output clamps to +0.9V (plus the common-
mode voltage of +0.6V), thus limiting the average maxi-
mum current from individual phases. The maximum
average-current-limit threshold for each phase is equal
to the maximum clamp voltage of the VEA divided by
the gain (18) of the current-sense amplifier. This allows
for accurate settings for the average maximum current
for each phase. Set the VEA gain using R_F and R_IN for
the amount of output voltage positioning required as
discussed in the Adaptive Voltage Positioning section
(Figures 3a and 3b).
Adaptive Voltage Positioning
Powering new-generation processors requires new
techniques to reduce cost, size, and power dissipation.
Voltage positioning reduces the total number of output
capacitors to meet a given transient response require-
ment. Setting the no-load output voltage slightly higher
than the output voltage during nominally loaded condi-
tions allows a larger downward voltage excursion when
the output current suddenly increases. Regulating at a
lower output voltage under a heavy load allows a larger
upward-voltage excursion when the output current sud-
denly decreases. A larger allowed, voltage-step excur-
sion reduces the required number of output capacitors
or allows for the use of higher ESR capacitors.
Voltage positioning and the ability to operate with multiple
reference voltages may require the output to regulate
away from a center value. Define the center value as the
voltage where the output drops (∆V_OUT/2) at one half the
maximum output current (Figure 5).
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