MAX15109 Datasheet, PDF (11/19 Pages) Maxim Integrated Products – High-Efficiency, 8A, Current-Mode Synchronous Step-Down Switching Regulator

High-Efficiency, 8A, Current-Mode Synchronous
Step-Down Switching Regulator with VID Control
Programmable Soft-Start (SS)
The IC utilizes a soft-start feature to slowly ramp up the
regulated output voltage to reduce input inrush current
during startup. Connect a capacitor from SS to PGND to
set the startup time. See the Output Voltage Transition
Timing section for capacitor selection details.
Error Amplifier
A high-gain error amplifier provides accuracy for the
voltage feedback loop regulation. Connect a compen-
sation network between COMP and SGND. See the
Compensation Design Guidelines section. The error
amplifier transconductance is 1.4mS. COMP clamp low
is set to 0.8V, just below the PWM ramp compensation
valley, helping COMP to rapidly return to the correct set
point during load and line transients.
PWM Comparator
The PWM comparator compares COMP voltage to the
current-derived ramp waveform (LX current to COMP
voltage transconductance value is 25A/V). To avoid
instability due to subharmonic oscillations when the duty
cycle is around 50% or higher, a compensation ramp
is added to the current-derived ramp waveform. The
compensation ramp slope (0.3V x 1MHz = 0.3V/Fs) is
equivalent to half of the inductor current down-slope in
the worst case (load 2A, current ripple 30% and maxi-
mum duty-cycle operation of 95%). The compensation
ramp valley is set to 1V.
Overcurrent Protection and Hiccup
When the converter output is connected to ground or the
device is overloaded, each high-side MOSFET current-
limit event (14A) turns off the high-side MOSFET and
turns on the low-side MOSFET. A 3-bit counter incre-
ments on each current-limit event. The counter is reset
after three consecutive events of high-side MOSFET
turn-on without reaching the current limit. If the current-
limit condition persists, the counter fills up reaching eight
events. The control logic then discharges SS, stops both
high-side and low-side MOSFETs and waits for a hiccup
period (1024 clock cycles) before attempting a new soft-
start sequence. The hiccup-mode also operates during
soft-start.
Thermal Shutdown Protection
The IC contains an internal thermal sensor that limits the
total power dissipation to protect it in the event of an
extended thermal fault condition. When the die tempera-
ture exceeds +160NC, the thermal sensor shuts down
the device, turning off the DC-DC converter to allow the
die to cool. After the die temperature falls by 25NC, the
device restarts, following the soft-start sequence.
Skip Mode Operation
The IC operates in skip mode. When in skip mode, LX
output becomes high impedance when the inductor
current falls below 0.7A. The inductor current does not
become negative. During a clock cycle, if the inductor
current falls below the 0.7A threshold (during off-time),
the low side turns off. At the next clock cycle, if the
output voltage is above the set point the PWM logic
keeps both high-side and low-side MOSFETs off. If
instead the output voltage is below the set point, the
PWM logic drives the high-side on for a minimum fixed
on-time (330ns). In this way, the system skips cycles,
reducing the frequency of operations, and switches only
as needed to service load at the cost of an increase in
output-voltage ripple. See the Skip Mode Frequency
and Output Ripple section for details. In skip mode,
power dissipation is reduced and efficiency improved at
light loads because the internal power MOSFETs do not
switch at every clock cycle.
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