Datasheet

the current limit and cause the fault latch to trip. To pro-
tect against this possibility, you can over design the cir-
cuit to tolerate:
where I
VALLEY(MAX)
is the maximum valley current
allowed by the current-limit circuit, including threshold
tolerance and on-resistance variation. The MOSFETs
must have a good-size heatsink to handle the overload
power dissipation.
Choose a Schottky diode (D
L
) with a forward voltage
low enough to prevent the low-side MOSFET body
diode from turning on during the dead time. Select a
diode that can handle the load current per phase dur-
ing the dead times. This diode is optional and can be
removed if efficiency is not critical.
Boost Capacitors
The boost capacitors (C
BST_
) must be selected large
enough to handle the gate-charging requirements of
the high-side MOSFETs. Typically, 0.1μF ceramic
capacitors work well for low-power applications driving
medium-sized MOSFETs. However, high-current appli-
cations driving large, high-side MOSFETs require boost
capacitors larger than 0.1μF. For these applications,
select the boost capacitors to avoid discharging the
capacitor more than 200mV while charging the high-
side MOSFETs’ gates:
where N is the number of high-side MOSFETs used for
one regulator, and Q
GATE
is the gate charge specified
in the MOSFET’s data sheet. For example, assume (2)
IRF7811W n-channel MOSFETs are used on the high
side. According to the manufacturer’s data sheet, a sin-
gle IRF7811W has a maximum gate charge of 24nC
(V
GS
= 5V). Using the above equation, the required
boost capacitance would be:
Selecting the closest standard value, this example
requires a 0.22μF ceramic capacitor.
Current-Balance Compensation (CCI)
The current-balance compensation capacitor (C
CCI
)
integrates the difference between the main and sec-
ondary current-sense voltages. The internal compensa-
tion resistor (R
CCI
= 200kΩ) improves transient
response by increasing the phase margin. This allows
the dynamics of the current-balance loop to be opti-
mized. Excessively large capacitor values increase the
integration time constant, resulting in larger current dif-
ferences between the phases during transients.
Excessively small capacitor values allow the current
loop to respond cycle-by-cycle, but can result in small
DC current variations between the phases. For most
applications, a 470pF capacitor from CCI to the switch-
ing regulator’s output works well.
Connecting the compensation network to the output
(V
OUT
) allows the controller to feed-forward the output-
voltage signal, especially during transients.
Voltage Positioning and
Loop Compensation
Voltage positioning dynamically lowers the output volt-
age in response to the load current, reducing the out-
put capacitance and processor’s power-dissipation
requirements. The controller uses a transconductance
amplifier to set the transient and DC output-voltage
droop (Figure 3) as a function of the load. This adjusta-
bility allows flexibility in the selected current-sense
resistor value or inductor DCR, and allows smaller cur-
rent-sense resistance to be used, reducing the overall
power dissipated.
Steady-State Voltage Positioning
Connect a resistor (R
FB
) between FB and V
OUT
to set
the DC steady-state droop (load line) based on the
required voltage-positioning slope (R
DROOP
):
where the effective current-sense resistance (R
SENSE
)
depends on the current-sense method (see the Current
Sense section), and the voltage-positioning amplifier’s
transconductance (G
m(FB)
) is typically 600μS as
defined in the Electrical Characteristics table. The con-
troller sums together the input signals of the current-
sense inputs (CSP_, CSN_).
When the inductors’ DCR is used as the current-sense
element (R
SENSE
= R
DCR
), each current-sense input
should include an NTC thermistor to minimize the tem-
perature dependence of the voltage-positioning slope.
R
R
RG
FB
DROOP
SENSE m FB
=
()
C
nC
mV
μF
BST_
.=
×
=
224
200
024
C
NQ
mV
BST
GATE
_
=
×
200
II
I
LOAD TOTAL VALLEY MAX
INDUCTOR
=+
⎛
⎝
⎜
⎞
⎠
⎟
η
()
Δ
2
()
()
=+η
TOTAL VALLEY MAX
LOAD MAX
I
I
LLIR
2
⎛
⎝
⎜
⎞
⎠
⎟
MAX17021/MAX17082/MAX17482
Dual-Phase, Quick-PWM Controllers for
IMVP-6+/IMVP-6.5 CPU Core Power Supplies
______________________________________________________________________________________ 45