MAX16956(2014) 查看數據表（PDF） - Maxim Integrated
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MAX16956 Datasheet PDF : 16 Pages
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MAX16956
36V, 300mA, Mini Buck Converter with
1.1µA IQ
Detailed Description
The MAX16956 is a small, current-mode buck converter
that features synchronous rectification and requires no
external compensation network. The device operates
from a 3.5V to 36V supply voltage and can deliver up
to 300mA output current. Frequency is fixed at 2.1MHz,
which allows for small external components, reduced
output ripple, and guarantees no AM-band interference.
The device offers fixed output voltages of 5V and 3.3V.
The device also offers adjustable output-voltage versions
that can be set between 1V and 10V by using an exter-
nal resistive divider. Voltage quality can be monitored
by observing the RESET signal. The device offers both
forced-PWM and skip mode, with ultra-low-quiescent cur-
rent of 1.1µA in skip mode.
DC-DC Converter Control Architecture
The device step-down converter uses a PWM peak current-
mode control scheme, with a load-line architecture. Peak
current-mode control provides several advantages over
voltage-mode control, including precise control of the induc-
tor current on a cycle-by-cycle basis, simpler compensa-
tion, and inherent compensation for line voltage variation.
An internal transconductance amplifier establishes an
integrated error voltage. The heart of the PWM control-
ler is an open-loop comparator: one input is the inte-
grated voltage-feedback signal; the other consists of the
amplified current-sense signal plus slope-compensation
ramp. Integrated high-side current sensing is used, which
reduces component count and layout risk by eliminating
the need to carefully route sensitive external signals.
Error-amplifier compensation is also integrated, once
again simplifying the power-supply designer’s task while
eliminating external components.
At each rising 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 (see the Current Limit /Short-Circuit Protection
section). During this on-time, current ramps up through
the inductor, storing energy in a magnetic field and
sourcing current to the output. The current-mode feed-
back system regulates the peak inductor current as a
function of the output-voltage error signal. During the
second-half of the cycle, the high-side MOSFET turns
off and the low-side MOSFET turns on. The inductor
releases the stored energy as the current ramps down,
providing current to the output. The output capacitor
stores charge when the inductor current exceeds the
required load current and discharges when the inductor
current is lower, smoothing the voltage across the load.
The device features load-line architecture to reduce the
output capacitance needed, potentially saving system
cost and size. The output voltage is positioned slightly
positive at no load, still within the tolerance window, to
take advantage of the fact that any load disturbance is
a load step only. This increases the amount of margin
available to the undershoot that occurs on a load step,
allowing a reduction in the required output capacitance.
As the load increases, a small but controlled amount
of load regulation (“load-line”) error occurs, so that at
heavier loads the voltage is positioned slightly below
nominal. This takes advantage of the fact that any load
disturbance is load released, increasing the amount of
margin available to the overshoot that occurs.
The device can operate in either forced-PWM or skip
mode. In forced-PWM mode, the converter maintains a
constant switching frequency, regardless of load, to allow
for easier filtering of the switching noise. The device
includes proprietary circuitry that dramatically reduces
quiescent current consumption in skip mode, improving
light-load efficiency. See the Forced PWM/Skip Modes
section for further details.
System Enable (EN)
An enable control input (EN) activates the device from its
low-power shutdown mode. EN is compatible with inputs
from automotive battery level down to 3.5V. The high-
voltage compatibility allows EN to be connected to SUP,
KEY/KL30, or the inhibit pin (INH) of a CAN transceiver.
Linear Regulator Output (BIAS)
The device includes a 5V linear regulator output (BIAS)
that provides power to the internal circuit blocks. Connect
a 1µF ceramic capacitor from BIAS to AGND. Do not load
this pin externally.
Undervoltage Lockout
When VBIAS drops below the undervoltage-lockout
(UVLO) level of VUVLO = 2.8V (typ), the device assumes
that the supply voltage is too low for proper operation, so
the UVLO circuitry inhibits switching. When VBIAS rises
above the UVLO rising threshold, the controller enters the
startup sequence and then resumes normal operation.
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