MAX1981A Maxim Integrated Products, MAX1981A Datasheet - Page 36

• Manufacturer Part Number: MAX1981A
• Description: (MAX1907A / MAX1981A) Quick-PWM Master Controllers
• Manufacturer: Maxim Integrated Products

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Quick-PWM Master Controllers for Voltage-
Positioned CPU Core Power Supplies (IMVP-IV)
where C
and I
(1A, typ).
Switching losses in the high-side MOSFET can become
a heat problem when maximum AC adapter voltages
are applied, due to the squared term in the C
f
chosen for adequate R
becomes extraordinarily hot when biased from
V
lower parasitic capacitance.
For the low-side MOSFET (N
dissipation always occurs at maximum input voltage:
The worst case for MOSFET power dissipation occurs
under heavy overloads that are greater than
I
the current limit and cause the fault latch to trip. To pro-
tect against this possibility, it is possible to “over
design” the circuit to tolerate:
where I
allowed by the current-limit circuit, including threshold
Figure 8. Voltage Positioning Gain
36
SW
LOAD(MAX)
IN(MAX)
PD N
switching-loss equation. If the high-side MOSFET
______________________________________________________________________________________
GATE
(
VALLEY(MAX)
RSS
L
I
, consider choosing another MOSFET with
LOAD
Re
is the peak gate-drive source/sink current
but are not quite high enough to exceed
sistive
is the reverse transfer capacitance of N
=
η
COMPARATOR
I
)
VALLEY MAX
=
ERROR
⎡
⎢
⎢
⎣
is the maximum “valley” current
1
−
DS(ON)
(
⎛
⎜
⎝
MAX1907A
MAX1981A
V
IN MAX
V
OUT
(
)
L
+
), the worst-case power
at low-battery voltages
⎛
⎜
⎝
)
I
LOAD MAX
⎞
⎟
⎠
⎤
⎥
⎥
⎦
⎛
⎜
⎝
I
LOAD
(
2
η
)
LIR
⎞
⎟
⎠
2
R
⎞
⎟
⎠
DS ON
✕
(
V
OAIN+
OAIN-
IN
)
FB
2
H
✕
tolerance and on-resistance variation. The MOSFETs
must have a relatively large heatsink to handle the over-
load power dissipation. Choose a Schottky diode (D1)
with a forward voltage low enough to prevent the low-
side MOSFET body diode from turning on during the
dead time. As a general rule, select a diode with a DC
current rating equal to 1/(3η) of the load current. This
diode is optional and can be removed if efficiency is
not critical.
Voltage positioning dynamically lowers the output volt-
age in response to the load current, reducing the
processor’s power dissipation. When the output is
loaded, an internal operational amplifier (Figures 2 and
8) increases the signal fed back to the master’s feed-
back input. The additional gain provided by the op amp
allows the use of low-value, current-sense resistors,
significantly reducing the power dissipated in the cur-
rent-sense resistors rather than connecting the feed-
back voltage directly to the current-sense resistor. The
load-transient response of this control loop is extremely
fast, yet well controlled, so the amount of voltage
change can be accurately confined within the limits
stipulated in the microprocessor power-supply guide-
lines. To understand the benefits of dynamically adjust-
ing the output voltage, see the Voltage Positioning and
Effective Efficiency section.
R
F
L
S
L
M
R
R
R
A
A
CS
R
CM
= R
CM
Setting Voltage Positioning
R
R
B
B
MASTER
SLAVE
BOARD
RESISTANCE