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PDF MAX8548EUB Data sheet ( Hoja de datos )
| Número de pieza | MAX8548EUB | |
| Descripción | Low-Cost / Wide Input Range / Step-Down Controllers with Foldback Current Limit | |
| Fabricantes | Maxim Integrated | |
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19-2795; Rev 0; 7/03
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
General Description
The 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.
Features
o 2.7V to 28V Input Range
o Foldback Short-Circuit Protection
o No Additional Bias Supply Needed
o 0.8V to 0.83 x VIN Output
o Up to 95% Efficiency
o Low-Cost External Components
o No Current-Sense Resistor
o All N-Channel MOSFET Design
o Adaptive Gate Drivers Eliminate Shoot-Through
o Lossless Overcurrent and Short-Circuit
Protection
o 300kHz Switching Frequency
(MAX8545/MAX8546)
o 100kHz Switching Frequency (MAX8548)
o Pin-Compatible with the MAX1967
o Thermal Shutdown
PART
MAX8545EUB
MAX8546EUB
MAX8548EUB
Ordering Information
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
10 µMAX
10 µMAX
10 µMAX
Set-Top Boxes
Graphic Card and Video
Supplies
Desktops and Desknotes
PCI Express Power
Supplies
Telecom Power Supplies
Applications
Notebook Docking
Station Supplies
Cable Modems and
Routers
Networking Power
Supplies
PART
MAX8545
MAX8546
MAX8548
Selector Guide
SWITCHING
FREQUENCY
300kHz
300kHz
100kHz
CURRENT-LIMIT
THRESHOLD
-320mV
-165mV
-320mV
Typical Operating Circuit
INPUT
2.7V TO 28V
OFF
ON
VCC VL
VIN BST
DH
MAX8545
MAX8546
MAX8548
LX
DL
COMP/
EN
GND
FB
OUTPUT
0.8V TO
0.9 x VIN
UP TO 6A
OPTIONAL
Pin Configuration appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1 page  
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
Typical Operating Characteristics (continued)
(VIN = VL = VCC = 5V, typical values are at TA = +25°C, unless otherwise noted.)
CHANGE IN OUTPUT VOLTAGE
vs. INPUT VOLTAGE
1.84
ILOAD = 6V
1.83
1.82
CHANGE IN OUTPUT VOLTAGE
vs. INPUT VOLTAGE
2.52
ILOAD = 6V
2.51
1.81 2.50
1.80
2.49
1.79
1.78
2.5
3.0 3.5 4.0
INPUT VOLTAGE (V)
4.5
2.48
10 12 14 16 18 20 22 24
INPUT VOLTAGE (V)
FREQUENCY vs. INPUT VOLTAGE
310
VOUT = 2.5V
NO LOAD
306 MAX8545/
MAX8546
302
298
294
290
2.70
7.76 12.82 17.88 22.94 28.00
INPUT VOLTAGE (V)
FREQUENCY vs. TEMPERATURE
310
VIN = 12V
VOUT = 2.5V
306 NO LOAD
MAX8545/
MAX8546
302
298
294
290
-40.00
-15.00 10.00 35.00 60.00
TEMPERATURE (°C)
85.00
LOAD TRANSIENT RESPONSE
MAX8545 toc14
VIN = 17V
VOUT = 2.5V
VOUT
AC COUPLED
100mV/div
IOUT
0 5A/div
40µs/div
_______________________________________________________________________________________ 5
5 Page 
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
A limitation of sensing current across a MOSFET’s on-
resistance is that the current-limit threshold is not accu-
rate since MOSFET RDS(ON) specifications are not
precise. This type of current limit provides a coarse level
of fault protection. It is especially suited when the input
source is already current-limited or otherwise protected.
Power MOSFET Selection
The MAX8545/MAX8546/MAX8548 drive two external,
logic-level, N-channel MOSFETs as the circuit switch-
ing elements. The key selection parameters are:
1) On-resistance (RDS(ON)): the lower, the better.
2) Maximum drain-to-source voltage (VDSS) should be
at least 10% higher than the input supply rail at the
high-side MOSFET’s drain.
3) Gate charges (Qg, Qgd, Qgs): the lower, the better.
Choose the MOSFETs with rated RDS(ON) at VGS = 4.5V
for an input voltage greater than 5V, and at VGS = 2.5V
for an input voltage less than 5.5V. For a good compro-
mise between efficiency and cost, choose the high-side
MOSFET (N1) that has conduction losses equal to the
switching losses at nominal input voltage and maximum
output current. For N2, make sure it does not spuriously
turn on due to a dV/dt caused by N1 turning on as this
would result in shoot-through current degrading the
efficiency. MOSFETs with a lower Qgd / Qgs ratio have
higher immunity to dV/dt.
MOSFET Power Dissipation
For proper thermal-management design, the power dis-
sipation must be calculated at the desired maximum
operating junction temperature, maximum output cur-
rent, and worst-case input voltage (for the low-side
MOSFET (N2) the worst case is at VIN(MAX), for the high-
side MOSFET (N1) the worst case can be either at
VIN(MIN) or VIN(MAX)). N1 and N2 have different loss
components due to the circuit operation. N2 operates as
a zero-voltage switch; therefore, the major losses are:
the channel conduction loss (PN2CC), the body-diode
conduction loss (PN2DC), and the gate-drive loss
(PN2DR).
PN2CC
=
1−
VOUT
VIN
×
I2LOAD
×
RDS(ON)
Use RDS(ON) at TJ(MAX).
PN2DC = 2 × ILOAD × VF × tdt × fS
where VF is the body-diode forward voltage drop, tdt is
the dead time between N1 and N2 switching transitions
(which is 30ns), and fS is the switching frequency.
Because of zero-voltage switch operation, the N2 gate-
drive losses are due to charging and discharging the
input capacitor, CISS. These losses are distributed
between the average DL gate driver’s pullup and pull-
down resistors and the internal gate resistance. The
RDL is typically 1.8Ω, and the internal gate resistance
(RGATE) of the MOSFET is typically 2Ω. The drive
power dissipated in N2 is given by:
( )PN2DR = CISS ×
VGS
2
×
fS
×
RGATE
RGATE + RDL
N1 operates as a duty-cycle control switch and has the
following major losses: the channel conduction loss
(PN1CC), the voltage and current overlapping switching
loss (PN1SW), and the drive loss (PN1DR). N1 does not
have a body-diode conduction loss because the diode
never conducts current.
( )PN1CC
=
VOUT
VIN
×
ILOAD
2 × RDS(ON)
Use RDS(ON) at TJ(MAX).
PN1SW
=
VIN
× ILOAD
×
fS
×
QGS + QGD
IGATE
where IGATE is the average DH high driver output-cur-
rent capability determined by:
IGATE(ON)
=
1
2
×
RDH
VL
+ RGATE
where RDH is the high-side MOSFET driver’s average
on-resistance (2.05Ω typ) and RGATE is the internal
gate resistance of the MOSFET (2Ω typ).
PN1DR
=
QGS
×
VGS
×
fS
×
RGATE
RDH + RGATE
where VGS ~ VL.
In addition to the losses above, allow about 20% more
for additional losses due to MOSFET output capaci-
tance and N2 body-diode reverse recovery charge dis-
sipated in N1. Refer to the MOSFET data sheet for
thermal resistance specifications to calculate the PC
board area needed. This information is essential to
maintain the desired maximum operating junction tem-
perature with the above calculated power dissipation.
To reduce EMI caused by switching noise, add a 0.1µF
ceramic capacitor from the high-side MOSFET drain to
the low-side MOSFET source or add resistors in series
______________________________________________________________________________________ 11
11 Page | ||
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| PDF Descargar | [ Datasheet MAX8548EUB.PDF ] | |
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