SC1480
POWER MANAGEMENT
Applications Information (Cont.)
Power Good Output
The high-side gate driver is equipped with turn-on soft
switching to reduce gate drive power dissipation. When
a DH turn-on is initiated the pull-up resistance is 10 Ohms.
This limits the peak high-side gate current before the
MOSFET is conducting current. The peak gate current
plays a large role in gate driver switching losses. When
the high-side MOSFET begins conducting, and LX starts
to rise, the pull-up resistance on DH changes to 2 Ohms.
Power good is an open-drain output and requires a pull-
up resistor. When the output voltage is 10% above or
below its set voltage, PGOOD gets pulled low. It is held
low until the output voltage returns to within 10% of the
output set voltage. PGOOD is also held low during start-
up and will not be allowed to transition high until the out-
put reaches 90% of its set voltage. There is a slight delay
built into the PGOOD circuit to prevent false transitions.
Output Overvoltage Protection
Design Procedure
When the output exceeds 10% of the its set voltage the
low-side MOSFET is latched on. It stays latched and the
SMPS is off until the enable input or POR is toggled. There
is a slight delay built into the OV protection circuit to pre-
vent false transitions.
Prior to any design of a switch mode power supply (SMPS)
for notebook computers, determination of input voltage,
load current, switching frequency and inductor ripple cur-
rent must be specified.
Input Voltage Range
Output Undervoltage Protection
The maximum input voltage (VINMAX) is determined by the
highest AC adaptor voltage. The minimum input voltage
(VINMIN) is determined by the lowest battery voltage after
accounting for voltage drops due to connectors, fuses
and battery selector switches.
When the output is 20% below its set voltage the output
is latched in a tristated condition, and the SMPS is off
until the enable input or POR is toggled. There is a slight
delay built into the UV protection circuit to prevent false
transitions.
Maximum Load Current
POR, UVLO and Softstart
There are two values of load current to consider. Con-
tinuous load current and peak load current. Continuous
load current has more to do with thermal stresses and
therefore drives the selection of input capacitors,
MOSFETs and commutation diodes. Whereas, peak load
current determines instantaneous component stresses
and filtering requirements such as, inductor saturation,
output capacitors and design of the current limit circuit.
An internal power-on reset (POR) occurs when VCCA ex-
ceeds 3V, resetting the fault latch and soft-start counter,
and preparing the PWM for switching. VCCA undervoltage
lockout (UVLO) circuitry inhibits switching and forces the
DL gate driver high until VCCA rises above 4.1V. At this
time the circuit will come out of UVLO and begin switch-
ing, and the softstart circuit being enabled, will progres-
sively limit the output current over a predetermined time
period. The ramp occurs in four steps: 25%, 50%, 75%
and 100%, thereby limiting the slew rate of the output
voltage. There is 100mV of hysteresis built into the UVLO
circuit and when the VCCA falls to 4.0V the output driv-
ers are shutdown and tristated.
Switching Frequency
Switching frequency determines the trade-off between
size and efficiency. Increased frequency increases the
switching losses in the MOSFETs, since losses are a func-
tion of VIN2. Knowing the maximum input voltage
and budget for MOSFET switches usually dictates where
the design ends up.
MOSFET Gate Drivers
The DH and DL drivers are optimized for driving moder-
ate-sized high-side, and larger low-side power MOSFETs.
An adaptive dead-time circuit monitors the DL output and
prevents the high-side MOSFET from turning on, until DL
is fully off, and conversely, monitors the DH output and
prevents the low-side MOSFET from turning on until DH
is fully off. Be sure there is low resistance and low induc-
tance between the DH and DL outputs to the gate of
each MOSFET.
Inductor Ripple Current
Low inductor values create higher ripple current, result-
ing in smaller size, but are less efficient because of the
high AC currents flowing through the inductor. Higher in-
ductor values do reduce the ripple current and are more
efficient, but are larger and more costly.
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