
Mode Selection (MODE)
The MAXM17503 features a MODE pin to configure the
device operating in PWM, PFM, or DCM control schemes.
The device operates in PFM mode at light loads if the
MODE pin is open. If the MODE pin connects to ground,
the device operates in constant-frequency PWM mode
at all loads. The device operates in constant-frequency
DCM mode at light loads when the MODE pin connects
to VCC. State changes of the MODE operation are only at
power-up and ignore during normal operation.
PWM Mode Operation
In PWM mode, the step-down controller is switching
a constant-frequency at all loads with a minimum sink
current limit threshold (-1.8A typ) at light load. The
PWM mode of operation gives lower efficiency at light
loads compared to PFM and DCM modes of operation.
However, the PWM mode of operation is useful in applica-
tions sensitive to switching frequency.
PFM Mode Operation
In PFM mode, the controller forces the peak inductor
current in order to feed the light loads and maintain high
efficiency. If the load is lighter than the average PFM
value, the output voltage will exceed 102.3% of the feed-
back threshold and the controller enters into a hibernation
mode, turning off most of the internal blocks. The device
exits hibernation mode, and starts switching again, once
the output voltage is discharged to 101.1% of the feedback
threshold. The device then begins the process of delivering
pulses of energy to the output repeatedly until it reaches
102.3% of the feedback threshold. In this mode, the
behavior resembles PWM operation (with occasional pulse
skipping), where the inductor current does not need to
reach the light-load level.
PFM mode offers the advantage of increased efficiency
at light loads due to a lower quiescent current drawn from
the supply. However, the output-voltage ripple is also
increased as compared to the PWM or DCM modes of
operation, and the switching frequency is not constant at
light loads.
DCM Mode Operation
DCM mode features constant frequency operation down
to lighter loads than PFM mode, accomplished by not
skipping pulses. DCM efficiency performance lies between
the PWM and PFM modes.
External Frequency Synchronization (SYNC)
The device can be synchronized by an external clock
signal on the SYNC pin. The external synchronization
clock frequency must be between 1.1 x fSW and 1.4 x fSW,
where fSW is the frequency programmed by the RT
resistor. The minimum external clock high pulse width
and amplitude should be greater than 50ns and 2.1V
respectively. The minimum external clock low pulse width
should be greater than 160ns, and the maximum external
clock low pulse amplitude should be less than 0.8V. Table 1
provides recommended synchronous frequency ranges
for desired output voltages. Connect the SYNC pin to
SGND if it is not used.
RESET Output
The device includes a RESET comparator to monitor the
output for undervoltage and overvoltage conditions. The
open-drain RESET output requires an external pullup
resistor from 10kΩ to 100kΩ to V
CC
pin or maximum 6V
voltage source. RESET goes high impedance after the
regulator output increases above 95% of the designed
nominal regulated voltage. RESET goes low when the
regulator output voltage drops below 92% of the nominal
regulated voltage. RESET also goes low during thermal
shutdown.
Thermal Fault Protection
The MAXM17503 features a thermal-fault protection
circuit. When the junction temperature rises above +165°C
(typ), a thermal sensor activates the fault latch, pulls down
the RESET output, and shuts down the regulator. The
thermal sensor restarts the controllers after the junction
temperature cools by 10°C (typ). The Soft-start resets
during thermal shutdown.
Power Dissipation and Output-Current Derating
The MAXM17503 output current needs to be derated
if the device needs to be operated in a high ambient-
temperature environment. The amount of current-derating
depends upon the input voltage, output voltage, and
ambient temperature. The derating curves in TOC43
from the Typical Operating Characteristics section can be
used as guidelines. The curves are based on simulating
thermal resistance model (ψJT), measuring thermal
resistance (ψTA), and measuring power dissipation
(PDMAX) on the bench.
MAXM17503 4.5V to 60V, 2.5A High-Efciency, DC-DC
Step-Down Power Module with Integrated Inductor
www.maximintegrated.com Maxim Integrated
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