MAX1981A Maxim Integrated Products, MAX1981A Datasheet - Page 32

• 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)
The output undervoltage protection (UVP) function is similar
to foldback current limiting, but employs a timer rather than
a variable current limit. If the MAX1907A/MAX1981A output
voltage is under 70% of the nominal value, the PWM is
latched off and won’t restart until SHDN is toggled or V
power is cycled below 1V. UVP is ignored during output
voltage transitions and remains blanked for an additional 32
clock cycles after the controller reaches the final DAC code
value.
UVP can be defeated through the NO FAULT test mode
(see the NO FAULT Test Mode section).
The MAX1907A/MAX1981A features a thermal fault-pro-
tection circuit. When the junction temperature rises
above 160°C, a thermal sensor activates the fault logic,
forces the DL low-side gate-driver high, and pulls the
DH high-side gate-driver low. This quickly discharges
the output capacitors, tripping the master controller’s
UVLO protection. Toggle SHDN or cycle V
below 1V to reactivate the controller after the junction
temperature cools by 15°C.
The OVP and UVP protection features can complicate
the process of debugging prototype breadboards
since there are (at most) a few milliseconds in which to
determine what went wrong. Therefore, a test mode is
provided to disable the OVP, UVP, and thermal shut-
down features, and clear the fault latch if it has been
set. The NO FAULT test mode is entered by forcing 12V
to 15V on SHDN.
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency
and inductor operating point (ripple-current ratio). The
primary design trade-off lies in choosing a good switch-
ing frequency and inductor operating point, and the fol-
lowing four factors dictate the rest of the design:
Input Voltage Range. The maximum value (V
must accommodate the worst-case, high-AC-adapter
voltage. The minimum value (V
the lowest input voltage after drops due to connectors,
fuses, and battery-selector switches. If there is a choice
at all, lower input voltages result in better efficiency.
Maximum Load Current. There are two values to con-
sider. The peak load current (I
32
______________________________________________________________________________________
Output Undervoltage Shutdown
Thermal Fault Protection
Design Procedure
NO FAULT Test Mode
IN(MIN)
LOAD(MAX)
) must account for
) determines
CC
IN(MAX)
power
CC
)
the instantaneous component stresses and filtering
requirements, and thus drives output capacitor selection,
inductor saturation rating, and the design of the current-
limit circuit. The continuous load current (ILOAD) deter-
mines the thermal stresses and thus drives the selection
of input capacitors, MOSFETs, and other critical heat-
contributing components. Modern notebook CPUs gen-
erally exhibit I
For multi-phase systems, each phase supports a frac-
tion of the load, depending on the current balancing.
When properly balanced, the load current is evenly dis-
tributed among each phase:
where η is the number of phases.
Switching Frequency. This choice determines the
basic trade-off between size and efficiency. The opti-
mal frequency is largely a function of maximum input
voltage, due to MOSFET switching losses that are pro-
portional to frequency and V
cy is also a moving target, due to rapid improvements
in MOSFET technology that are making higher frequen-
cies more practical.
Setting Slave On-Time. The constant on-time control
algorithm in the master results in a nearly constant
switching frequency despite the lack of a fixed-frequen-
cy clock generator. In the slave, the high-side switch
on-time is inversely proportional to V+, and directly pro-
portional to the compensation voltage (V
where K set by the TON pin-strap connection (Table 3).
Inductor Operating Point. This choice provides trade-
offs between size vs. efficiency and transient response
vs. output noise. Low inductor values provide better
transient response and smaller physical size, but also
result in lower efficiency and higher output noise due to
increased ripple current. The minimum practical induc-
tor value is one that causes the circuit to operate at the
edge of critical conduction (where the inductor current
just touches zero with every cycle at maximum load).
Inductor values lower than this grant no further size-
reduction benefit. The optimum operating point is usu-
ally found between 20% and 50% ripple current.
I
LOAD SLAVE
LOAD
(
= I
t
ON
)
LOAD(MAX)
=
=
I
LOAD MASTER
K
⎛
⎜
⎝
V
COMP
(
IN
V
IN
2
. The optimum frequen-
✕
⎞
⎟
⎠
80%.
)
=
I
LOAD
COMP
η
):