Rainbow-electronics MAX17031 Manual de usuario Pagina 19
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Power-Good Outputs (PGOOD)
and Fault Protection
PGOOD is the open-drain output that continuously
monitors both output voltages for undervoltage and
overvoltage conditions. PGOOD is actively held low in
shutdown (ON1 or ON2 = GND), during soft-start, and
soft-shutdown. Approximately 20µs (typ) after the soft-
start terminates, PGOOD becomes high impedance as
long as both output voltages exceed 85% of the nomi-
nal fixed-regulation voltage. PGOOD goes low if the
output voltage drops 15% below the regulation voltage,
or if the SMPS controller is shut down. For a logic-level
PGOOD output voltage, connect an external pullup
resistor between PGOOD and the logic power supply.
A 100kΩ pullup resistor works well in most applications.
Overvoltage Protection (OVP)
When the output voltage rises 15% above the fixed-
regulation voltage, the controller immediately pulls
PGOOD low, sets the overvoltage fault latch, and imme-
diately pulls the respective DL_ high—clamping the
output fault to GND. Toggle either ON1 or ON2 input, or
cycle V
CC
power below its POR threshold to clear the
fault latch and restart the controller.
Undervoltage Protection (UVP)
When the output voltage drops 30% below the fixed-
regulation voltage, the controller immediately pulls the
PGOOD low, sets the undervoltage fault latch, and
begins the shutdown sequence. After the output volt-
age drops below 0.1V, the synchronous rectifier turns
on, clamping the output to GND regardless of the out-
put voltage. Toggle either ON1 or ON2 input, or cycle
V
CC
power below its POR threshold to clear the fault
latch and restart the controller.
Thermal-Fault Protection (T
SHDN
)
The MAX17031 features a thermal-fault protection cir-
cuit. When the junction temperature rises above
+160°C, a thermal sensor activates the fault latch, pulls
PGOOD low, enables the 10Ω discharge circuit, and
disables the controller—DH and DL pulled low. Toggle
ONLDO or cycle IN power to reactivate the controller
after the junction temperature cools by 15°C.
Design Procedure
Firmly establish the input-voltage range and maximum
load current before choosing an inductor operating
point (ripple-current ratio). The primary design goal is
choosing a good inductor operating point, and the fol-
lowing three 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 battery voltage after
drops due to connectors, fuses, and battery-selec-
tor switches. If there is a choice at all, lower input
voltages result in better efficiency.
• Maximum Load Current: There are two values to
consider. The peak load current (I
LOAD(MAX)
) deter-
mines the instantaneous component stresses and fil-
tering requirements and thus drives output capacitor
selection, inductor saturation rating, and the design of
the current-limit circuit. The continuous load current
(I
LOAD
) determines the thermal stresses and thus dri-
ves the selection of input capacitors, MOSFETs, and
other critical heat-contributing components.
MAX17031
Dual Quick-PWM Step-Down Controller with Low-
Power LDO and RTC Regulator for MAIN Supplies
______________________________________________________________________________________ 19
MODE CONTROLLER STATE DRIVER STATE
Shutdown (ON_ = High to Low)
Output UVP (Latched)
Voltage soft-shutdown initiated. Internal error-amplifier
target slowly ramped down to GND and output actively
discharged (automatically enters forced-PWM mode).
DL driven high and DH pulled low
after soft-shutdown completed
(output < 0.1V).
Output OVP (Latched)
Controller shuts down and EA target internally slewed
down. Controller remains off until ON_ toggled or V
CC
power cycled.
DL immediately
driven high,
DH pulled low.
UVLO (V
CC
Falling-Edge)
Thermal Fault (Latched)
SMPS controller disabled (assuming ON_ pulled high),
10 output discharge active.
DL and DH pulled low.
UVLO (V
CC
Rising Edge)
SMPS controller disabled (assuming ON_ pulled high),
10 output discharge active.
DL driven high,
DH pulled low.
V
CC
Below POR SMPS inactive, 10 output discharge active.
DL driven high,
DH pulled low.
Table 3. Fault Protection and Shutdown Operation Table
- MAX17031 1
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