MAX1980 Datasheet, PDF (18/33 Pages) Maxim Integrated Products – Quick-PWM Slave Controller with Driver Disable for Multiphase DC-DC Converter

Quick-PWM Slave Controller with
Driver Disable for Multiphase DC-DC Converter
Undervoltage Lockout
During startup, the V_CC undervoltage lockout (UVLO)
circuitry forces the DL and the DH gate drivers low,
inhibiting switching until an adequate supply voltage is
reached. Once V_CC rises above 3.75V, valid transitions
detected at the trigger input initiate a corresponding
on-time pulse (see the On-Time Control and Active
Current Balancing section). To ensure correct startup,
the MAX1980 slave controller’s undervoltage lockout
voltage must be lower than the master controller’s
undervoltage lockout voltage.
If the V_CC voltage drops below 3.75V, it is assumed that
there is not enough supply voltage to make valid deci-
sions. To protect the output from overvoltage faults, DL
and DH are forced low, effectively disabling the
MAX1980.
Thermal-Fault Protection
The MAX1980 features a thermal-fault-protection circuit.
When the junction temperature rises above +160°C, a
thermal sensor activates the standby logic, which pulls
DL and DH low. The thermal sensor reactivates the
slave controller after the junction temperature cools by
15°C.
Design Procedure
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_IN(MAX))
must accommodate the worst-case high AC adapter
voltage. The minimum value (V_IN(MIN)) must account for
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_LOAD(MAX)) determines
the instantaneous component stresses and filtering
requirements, and thus drives output capacitor selec-
tion, inductor saturation rating, and the design of the
current-limit circuit. The continuous load current (I_LOAD)
determines the thermal stresses and thus drives the
selection of input capacitors, MOSFETs, and other criti-
cal heat-contributing components. Modern notebook
CPUs generally exhibit I_LOAD = I_LOAD(MAX) ✕ 80%.
For multiphase systems, each phase supports a frac-
tion of the load, depending on the current balancing.
The highly accurate current sensing and balancing
implemented by the MAX1980 slave controller evenly
distributes the load among each phase:
I_LOAD(SLAVE)
=
I_LOAD(MASTER)
=
I_LOAD
η
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_IN². The optimum frequen-
cy also is a moving target, due to rapid improvements
in MOSFET technology that are making higher frequen-
cies more practical.
Setting Switch 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 propor-
tional to the compensation voltage (V_COMP):
t_ON
=
K


V_COMP
V_IN


where K is set by the TON pin-strap connection (Table 3).
Set the nominal on time in the slave to match the on
time in the master. An exact match is not necessary
because the MAX1980 have wide t_ON adjustment
ranges (±40%). For example, if t_ON in the master is set
to 250kHz, the slave can be set to either 200kHz or
300kHz and still achieve good performance. Care
should be taken to ensure that the COMP voltage
remains within its output voltage range (0.42V to 2.80V).
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.
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