MAX5066
Configurable, Single-/Dual-Output, Synchronous
Buck Controller for High-Current Applications
20 ______________________________________________________________________________________
Output-Voltage Setting
The output voltage is set by the combination of resistors
R1, R2, and R
F
as described in the Voltage Error Amplifier
section. First select a value for resistor R2. Then calculate
the value of R1 from the following equation:
where V
OUT(NL)
is the voltage at no load. Then find the
value of R
F
from the following equation:
where ∆V
OUT
is the allowable drop in voltage from no
load to full load. R
F
is R8 and R9, R1 is R4 and R6, R2
is R5 and R7 in Figure 6.
Compensation
The MAX5066 uses an average current-mode control
scheme to regulate the output voltage (see Figure 2).
The main control loop consists of an inner current loop
and an outer voltage loop. The voltage error amplifier
(VEA1 and VEA2) provides the controlling voltage for
the current loop in each phase. The output inductor is
“hidden” inside the inner current loop. This simplifies
the design of the outer voltage control loop and also
improves the power-supply dynamics. The objective of
the inner current loop is to control the average inductor
current. The gain-bandwidth characteristic of the cur-
rent loop can be tailored for optimum performance by
the compensation network at the output of the current-
error amplifier (CEA1 or CEA2). Compared with peak
current-mode control, the current-loop gain crossover
frequency, f
C
, can be made approximately the same,
but the gain at low frequencies is much higher. This
results in the following advantages over peak current-
mode control.
1) The average current tracks the programmed cur-
rent with a high degree of accuracy.
2) Slope compensation is not required, but there is a
limit to the loop gain at the switching frequency in
order to achieve stability.
3) Noise immunity is excellent.
4) The average current-mode method can be used to
sense and control the current in any circuit branch.
For stability of the current loop, the amplified inductor-
current downslope at the negative input of the PWM
comparator (CPWM1 and CPWM2) must not exceed
the ramp slope at the comparator’s positive input. This
puts an upper limit on the current-error amplifier gain at
the switching frequency. The inductor current downs-
lope is given by V
OUT
/L where L is the value of the
inductor (L1 and L2 in Figure 6) and V
OUT
is the output
voltage. The amplified inductor current downslope at
the negative input of the PWM comparator is given by:
where R
SENSE
is the current-sense resistor (R1 and R2
in Figure 6) and g
M
x R
CF
is the gain of the current-error
amplifier (CEA_) at the switching frequency. The slope
of the ramp at the positive input of the PWM comparator
is 2V x f
SW
. Use the following equation to calculate the
maximum value of R
CF
(R14 or R15 in Figure 6).
The highest crossover frequency f
CMAX
is given by:
or alternatively:
Equation (1) can now be rewritten as:
In practical applications, pick the crossover frequency
(f
C
) in the range of:
First calculate R
CF
in equation 2 above. Calculate C
CF
such that:
where C
CF
is C10 and C12 in Figure 6.
