MAX8545EUB ,Low-Cost / Wide Input Range / Step-Down Controllers with Foldback Current Limitapplications. No Additional Bias Supply NeededThey drive low-cost N-channel MOSFETs for both thehi ..
MAX8545EUB+T ,Low-Cost, Wide Input Range, Step-Down Controllers with Foldback Current LimitMAX8545/MAX8546/MAX854819-2795; Rev 2; 6/07Low-Cost, Wide Input Range, Step-DownControllers with Fo ..
MAX8546EUB ,Low-Cost / Wide Input Range / Step-Down Controllers with Foldback Current Limitapplications. It drives low-cost N-channel MOSFETs for both the high-side switch and synchronous re ..
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MAX8546EUB ,Low-Cost / Wide Input Range / Step-Down Controllers with Foldback Current LimitTable of Contents I. ........Device Description V. ........Quality Assurance Information II ..
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MAX8545EUB
Low-Cost / Wide Input Range / Step-Down Controllers with Foldback Current Limit
General DescriptionThe MAX8545/MAX8546/MAX8548 are voltage-mode
pulse-width-modulated (PWM), step-down DC-DC con-
trollers ideal for a variety of cost-sensitive applications.
They drive low-cost N-channel MOSFETs for both the
high-side switch and synchronous rectifier, and require
no external current-sense resistor. These devices can
supply output voltages as low as 0.8V.
The MAX8545/MAX8546/MAX8548 have a wide 2.7V to
28V input range, and do not need any additional bias
voltage. The output voltage can be precisely regulated
from 0.8V to 0.83 x VIN. These devices can provide effi-
ciency up to 95%. Lossless short-circuit and current-limit
protection is provided by monitoring the RDS(ON)of the
low-side MOSFET. The MAX8545 and MAX8548 have a
current-limit threshold of 320mV, while the MAX8546 has
a current-limit threshold of 165mV. All devices feature
foldback-current capability to minimize power dissipation
under short-circuit condition. Pulling the COMP/EN pin
low with an open-collector or low-capacitance, open-
drain device can shut down all devices.
The MAX8545/MAX8546 operate at 300kHz and the
MAX8548 operates at 100kHz. The MAX8545/
MAX8546/MAX8548 are compatible with low-cost alu-
minum electrolytic capacitors. Input undervoltage lock-
out prevents proper operation under power-sag
operations to prevent external MOSFETs from overheat-
ing. Internal soft-start is included to reduce inrush cur-
rent. These devices are offered in space-saving 10-pin
µMAX packages.
Applications
Features2.7V to 28V Input RangeFoldback Short-Circuit ProtectionNo Additional Bias Supply Needed0.8V to 0.83 x VINOutputUp to 95% EfficiencyLow-Cost External ComponentsNo Current-Sense ResistorAll N-Channel MOSFET DesignAdaptive Gate Drivers Eliminate Shoot-ThroughLossless Overcurrent and Short-Circuit
Protection300kHz Switching Frequency
(MAX8545/MAX8546)100kHz Switching Frequency (MAX8548)Pin-Compatible with the MAX1967Thermal Shutdown
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
Typical Operating Circuit
Ordering Information
Pin Configuration appears at end of data sheet.19-2795; Rev 0; 7/03
Selector GuideSet-Top Boxes
Graphic Card and Video
Supplies
Desktops and Desknotes
PCIExpress Power
Supplies
Telecom Power Supplies
Notebook Docking
Station Supplies
Cable Modems and
Routers
Networking Power
Supplies
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICSStresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
(All voltages referenced to GND unless otherwise noted.)
VINto GND ............................................................-0.3V to +30V
VCCto GND .............................-0.3V, lower of 6V or (VL + 0.3V)
FB to GND................................................................-0.3V to +6V
BST to GND............................................................-0.3V to +36V
VL, DL, COMP to GND ..............................-0.3V to (VCC+ 0.3V)
BST to LX..................................................................-0.3V to +6V
DH to LX....................................................-0.3V to (VBST+ 0.3V)
VL Short to GND ......................................................................5s
LX to GND ......................................................................0 to 30V
Input Current (any pin).....................................................±50mA
Continuous Power Dissipation (TA= +70°C)
10-Pin µMAX (derate 5.6mW/°C above +70°C)..........444mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
ELECTRICAL CHARACTERISTICS (continued)(VIN= VL= VCC= 5V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
remains on.
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
Typical Operating Characteristics(VIN= VL= VCC= 5V, typical values are at TA = +25°C, unless otherwise noted.)
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
CHANGE IN OUTPUT VOLTAGE
vs. INPUT VOLTAGEMAX8545/46/48 toc10
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
CHANGE IN OUTPUT VOLTAGE
vs. INPUT VOLTAGE
MAX8545/46/48 toc11
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)202224161412
FREQUENCY vs. INPUT VOLTAGE
MAX8545/46/48 toc12
INPUT VOLTAGE (V)
FREQUENCY (kHz)
FREQUENCY vs. TEMPERATURE
MAX8545/46/48 toc13
TEMPERATURE (°C)
FREQUENCY (kHz)
VOUT
AC COUPLED
100mV/div
IOUT
5A/div
40µs/div
LOAD TRANSIENT RESPONSEMAX8545 toc14
ypical Operating Characteristics (continued)(VIN= VL= VCC= 5V, typical values are at TA = +25°C, unless otherwise noted.)
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limitypical Operating Characteristics (continued)(VIN= VL= VCC= 5V, typical values are at TA = +25°C, unless otherwise noted.)
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
Functional Diagram
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
Detailed DescriptionThe MAX8545/MAX8546/MAX8548 are BiCMOS switch-
mode power-supply controllers designed to implement
simple, buck-topology regulators in cost-sensitive
applications. The main power-switching circuit consists
of two N-channel MOSFETs, an inductor, and input/out-
put filter capacitors. An all N-channel synchronous-rec-
tified design provides high efficiency at reduced cost.
These devices have an internal 5V linear regulator that
steps down the input voltage to supply the IC and the
gate drivers. The low-side-switch gate driver is directly
powered from the 5V regulator (VL), while the high-
side-switch gate driver is indirectly powered from VL
plus an external diode-capacitor boost circuit.
Current-Limit and
Short-Circuit ProtectionThe MAX8545/MAX8546/MAX8548 employ a valley cur-
rent-sensing algorithm that uses the RDS(ON)of the low-
side N-channel MOSFET to sense the current. This
eliminates the need for an external sense resistor usually
placed in series with the output. The voltage measured
across the low-side MOSFET’s RDS(ON)is compared to
a fixed -320mV reference for the MAX8545/MAX8548
and a fixed -165mV reference for the MAX8546. The cur-
rent limit is given by the equations below:
Aside from current limiting, these devices feature fold-
back short-circuit protection. This feature is designed
to reduce the current limit by 80% as the output voltage
drops to 0V.
MOSFET Gate DriversThe DH and DL drivers are optimized for driving N-
channel MOSFETs with low gate charge. An adaptive
dead-time circuit monitors the DL output and prevents
the high-side MOSFET from turning on until the low-side
MOSFET is fully off. There must be a low-resistance,
low-inductance connection from the DL driver to the
MOSFET gate for the adaptive dead-time circuit to work
properly. Otherwise, the sense circuitry in the MAX8545/
MAX8546/MAX8548 may detect the MOSFET gate as off
while there is actually charge left on the gate. Use very
short, wide traces measuring no less than 50 to 100 mils
wide if the MOSFET is 1 inch away from the MAX8545/
MAX8546/MAX8548. The same type of adaptive dead-
time circuit monitors the DH off edge. The same recom-
mendations apply for the gate connection of the
high-side MOSFET.
The internal pulldown transistor that drives DL low is
robust, with a 1.1Ω(typ) on-resistance. This helps pre-
vent DL from being pulled up due to capacitive cou-
pling from the drain to the gate of the low-side
synchronous-rectifier MOSFET during the fast rise time
of the LX node.
Soft-StartThe MAX8545/MAX8546/MAX8548 feature an internally
set soft-start function that limits inrush current. It accom-
plishes this by ramping the internal reference input to the
controller’s transconductance error amplifier from 0 to
the 0.8V reference voltage. The ramp time is 1024 oscil-
lator cycles for the MAX8548 and 2048 oscillator cycles
for the MAX8545/MAX8546. At the nominal 100kHz and
300kHz switching rate, the soft-start ramp is approxi-
mately 10.2ms and 6.8ms, respectively.
High-Side Gate-Drive Supply (BST)A flying-capacitor boost circuit generates gate-drive volt-
age for the high-side N-channel MOSFET. The flying
capacitor is connected between the BST and LX nodes.
On startup, the synchronous rectifier (low-side MOSFET)
forces LX to ground and charges the boost capacitor to
VL. On the second half-cycle, the MAX8545/MAX8546/
MAX8548 turn on the high-side MOSFET by closing an
internal switch between BST and DH. This provides the
necessary gate-to-source voltage to drive the high-side
MOSFET gate above its source at the input voltage.
Internal 5V Linear RegulatorAll MAX8545/MAX8546/MAX8548 functions are internally
powered from an on-chip, low-dropout 5V regulator (VL).
These devices have a maximum input voltage (VIN) of
28V. Connect VCCto VLthrough a 10Ωresistor and
bypass VCCto GND with a 0.1µF ceramic capacitor. The
VIN-to-VL dropout voltage is typically 140mV, so when VIN
is less than 5.5V, VL is typically VIN- 140mV.
The internal linear regulator can source a minimum of
25mA and a maximum of approximately 40mA to supply
power to the IC low-side and high-side MOSFET drivers.
Duty-Cycle Limitations for
Low VOUT/VINRatiosThe MAX8545/MAX8546/MAX8548’s output voltage is
adjustable down to 0.8V. However, the minimum duty
cycle can limit the ability to supply low-voltage outputs
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limitfrom high-voltage inputs. With high input voltages, the
required duty factor is approximately:
where RDS(ON)x ILOADis the voltage drop across the
synchronous rectifier. Therefore, the maximum input
voltage (VIN(DFMAX)) that can supply a given output
voltage is:
If the circuit cannot attain the required duty cycle dic-
tated by the input and output voltages, the output volt-
age still remains in regulation. However, there may be
intermittent or continuous half-frequency operation as
the controller attempts to lower the average duty cycle
by deleting pulses. This can increase output voltage
ripple and inductor current ripple, which increases
noise and reduces efficiency. Furthermore, circuit sta-
bility is not guaranteed.
Applications Information
Design Procedures1)
Input Voltage Range. The maximum value(VIN(MAX)) must accommodate the worst-case high
input voltage. The minimum value (VIN(MIN)) must
account for the lowest input voltage after drops due
to connectors, fuses, and switches are considered.
In general, lower input voltages provide the best
efficiency.
2)
Maximum Load Current. There are two currentvalues to consider. Peak load current (ILOAD(MAX))
determines the instantaneous component stresses
and filtering requirements and is key in determining
output capacitor requirements. ILOAD(MAX)also
determines the required inductor saturation rating.
Continuous load current (ILOAD) determines the
thermal stresses, input capacitor, and MOSFETs,
as well as the RMS ratings of other heat-contribut-
ing components such as the inductor.
3)
Inductor Value. This choice provides tradeoffsbetween size, transient response, and efficiency.
Higher inductance value results in lower inductor
ripple current, lower peak current, lower switching
losses, and, therefore, higher efficiency at the cost
of slower transient response and larger size. Lower
inductance values result in large ripple currents,
smaller size, and poor efficiency, while also provid-
ing faster transient response.
Setting the Output VoltageAn output voltage between 0.8V and (0.83 x VIN) can
be configured by connecting FB to a resistive divider
between the output and GND (see Figures 1 and 2).
Select resistor R4 in the 1kΩto 10kΩrange. R3 is then
given by:
where VFB= +0.8V.
Inductor SelectionDetermine an appropriate inductor value with the fol-
lowing equation:
where LIR is the ratio of inductor ripple current to aver-
age continuous maximum load current. Choosing LIR
between 20% to 40% results in a good compromise
between efficiency and economy. Choose a low-core-
loss inductor with the lowest possible DC resistance.
Ferrite-core-type inductors are often the best choice for
performance; however, the MAX8548’s low switching
frequency also allows the use of powdered iron core
inductors in ultra-low-cost applications where efficiency
is not critical. With any core material, the core must be
large enough not to saturate at the peak inductor cur-
rent (IPEAK).
Setting the Current LimitThe MAX8545/MAX8546/MAX8548 provide valley cur-
rent limit by sensing the voltage across the external
low-side MOSFET. The minimum current-limit threshold
voltage is -280mV for the MAX8545/MAX8548 and
-140mV for the MAX8546. The MOSFET on-resistance
required to allow a given peak inductor current is:
where IVALLEY= ILOAD(MAX)x (1 - LIR / 2), and
RDS(ON)MAXis the maximum on-resistance of the low-
side MOSFET at the maximum operating junction
temperature.
