MAX16955
36V, 1MHz Step-Down Controller
with Low Operating Current
or driven by an external clock, the MAX16955 operates
in skip mode during soft-start and transitions to fixed-
frequency PWM mode after soft-start is over.
Error Detection and Fault Behavior
Several error-detection mechanisms prevent damage to
the MAX16955 and the application circuit:
? Overcurrent protection
? Output overvoltage protection
? Undervoltage lockout at BIAS
threshold. The PGOOD output is open drain and should
be pulled up with an external resistor to the supply volt-
age of the logic input it drives. This voltage should not
exceed 6V. A 10k Ω pullup resistor works well in most
applications. PGOOD can sink up to 4mA of current
while low.
PGOOD asserts low during the following conditions:
? Standby mode
? Undervoltage with V OUT below 90% (typ) its set
value
?
?
Power-good detection of the output voltage
Overtemperature protection of the IC
?
Overvoltage with V OUT above 111% (typ) its set
value
Overcurrent Protection
The MAX16955 provides cycle-by-cycle current limiting
as long as the FB voltage is greater than 0.7V (i.e., 70%
of the regulated output voltage). If the output voltage
drops below 70% of the regulation point due to overcur-
rent event, 16 consecutive current-limit events initiate
restart. If the overcurrent is still present during restart,
the MAX16955 shuts down and initiates restart. This
automatic restart continues until the overcurrent condi-
tion disappears. If the overcurrent condition disappears
at any restart attempt, the device enters the normal
soft-start routine.
If the output is shorted through a long wire, output volt-
age can fall significantly below ground before reaching
the overcurrent limit. Under this condition, the MAX16955
stops switching and initiates restart as soon as output
drops to 20% of its regulation point.
Output Overvoltage Protection
The MAX16955 features an internal output overvoltage
protection. If V OUT increases by 13% (typ) of the intended
regulation voltage, the high-side MOSFET turns off and
the low-side MOSFET turns on. The low-side MOSFET
stays on until V OUT goes back into regulation. Once V OUT
is in regulation, the normal switching cycles continue.
Undervoltage Lockout (UVLO)
The BIAS input undervoltage lockout (UVLO) circuitry
inhibits switching if the 5V bias supply (BIAS) is below
its UVLO threshold, 3.1V (typ). If the BIAS voltage
drops below the UVLO threshold, the controller stops
switching and turns off both high-side and low-side
gate drivers until the BIAS voltage recovers.
Power-Good Detection (PGOOD)
The MAX16955 includes a power-good comparator
with added hysteresis to monitor the step-down con-
troller ’s output voltage and detect the power-good
Maxim Integrated
The power-good levels are measured at FB if a feed-
back divider is used. If the MAX16955 is used in 5V
mode with FB connected to BIAS, OUT is used as a
feedback path for voltage regulation and power-good
determination.
Overtemperature Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX16955. When the junction temperature
exceeds +175°C (typ), an internal thermal sensor shuts
down the step-down controller, allowing the IC to cool.
The thermal sensor turns on the IC again after the junc-
tion temperature cools by 15°C and the output voltage
has dropped below 1.25V (typ).
A continuous overtemperature condition can cause
on-/off-cycling of the device.
Fixed-Frequency, Current-Mode
PWM Controller
The MAX16955’s step-down controller uses a PWM,
current-mode control scheme. An internal transconduc-
tance amplifier establishes an integrated error voltage.
The heart of the PWM controller is an open-loop com-
parator that compares the integrated voltage-feedback
signal against the amplified current-sense signal plus
the slope compensation ramp, which are summed into
the main PWM comparator to preserve inner-loop sta-
bility and eliminate inductor stair casing. At each falling
edge of the internal clock, the high-side MOSFET turns
on until the PWM comparator trips, the maximum duty
cycle is reached, or the peak current limit is reached.
During this on-time, current ramps up through the
inductor, storing energy in its magnetic field and sourc-
ing current to the output. The current-mode feedback
system regulates the peak inductor current as a func-
tion of the output-voltage error signal. The circuit acts
as a switch-mode transconductance amplifier and elim-
inates the influence of the output LC filter double pole.
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