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PDF MAX8541EEE Data sheet ( Hoja de datos )
| Número de pieza | MAX8541EEE | |
| Descripción | Synchronizable / High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies | |
| Fabricantes | Maxim Integrated | |
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19-2830; Rev 0; 4/03
EVAALVUAAILTAIOBNLEKIT
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
General Description
The MAX8540/MAX8541 pulse-width-modulated (PWM)
controllers for forward/flyback isolated DC-to-DC convert-
ers provide maximum flexibility to power-supply design-
ers and reduce external component count. These
controllers allow common PC board layout for current-
mode (MAX8540) and voltage-mode (MAX8541) designs.
Both controllers feature adjustable switching frequency
and external synchronization from 200kHz to 1MHz.
Active-high and active-low enable, undervoltage pro-
tection (UVP), and overvoltage protection (OVP) reduce
external component count.
Maximum duty cycle is adjustable, and the feed-forward
function scales the maximum duty cycle with input volt-
age to limit the maximum volt-seconds applied to the
transformer primary. The MAX8540 allows the user to
select the value of slope compensation to further opti-
mize magnetics design. The MAX8541 features user-
adjustable ramp magnitude for the PWM comparator.
A cycle-by-cycle current-limit function controls the peak
primary current during overload and short circuit. Both
controllers can be set to latch off or to hiccup when a
short circuit is detected. The number of current-limited
cycles to initiate the hiccup mode and number of
cycles skipped are user selectable to allow startup with
high-capacitance loads and protect the converter
against sustained short circuits.
The MAX8540/MAX8541 feature 9Ω internal gate dri-
vers for low-power applications and are compatible
with external gate drivers for high-power applications.
Both devices are available in the space-saving 16-pin
QSOP package.
The MAX8541 EV kit evaluates the MAX8540 and the
MAX8541 to speed designs. Design examples are also
available in application notes: 50W Voltage-Mode
Forward Converter Design with the MAX8541 and 50W
Current-Mode Forward Converter Design with the
MAX8540.
Applications
Isolated DC-to-DC Modules (Bricks)
Cellular Base Stations
Telecom and Network Systems
High-Performance Off-Line AC/DC Converters
Functional Diagrams and Typical Operating Circuit appear
at end of data sheet.
Features
o 200kHz to 1MHz Adjustable Switching Frequency
o Synchronization to External Clock
o Programmable, Constant Maximum Volt-Second
Simplifies Transformer Design
o Programmable Hiccup/Skip Cycles or Latch-Mode
Protection
o Clean Startups with High-Capacitance Loads
o Programmable Under/Overvoltage Protection
o Current-Mode, Adjustable Slope Compensation
(MAX8540)
o Voltage-Mode, Adjustable Ramp Magnitude
(MAX8541)
o 10mA, 5V Regulator
o 75mV to 1.25mV Adjustable Current Limit
Reduces External Components
o Internal Gate Driver for Low-Power Applications
o External Gate Driver for High-Power Applications
PART
MAX8540EEE
MAX8541EEE
Ordering Information
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
16 QSOP
16 QSOP
Pin Configurations
TOP VIEW
REF 1
OV 2
UV 3
FREQ/SYNC 4
SS 5
MAXDTY 6
EN 7
SCOMP 8
MAX8540EEE
16 VCC
15 DRV
14 GND
13 SKTON
12 CS
11 SKTOFF
10 ILIM
9 OPTO
QSOP
Pin Configurations continued 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  
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
ELECTRICAL CHARACTERISTICS (continued)
(VVCC = 12V, CREF = 1µF, VUV = VOV = 2V, RFREQ/SYNC = 32.4kΩ, CSS = 0.1µF, RMAXDTY = 97.6kΩ, VEN = 0, RSCOMP = 68kΩ,
RPRAMP = 25kΩ, CSKTON = 0.01µF, CSKTOFF = 0.1µF, VOPTO = 2V, VILIM = 4V, VCS = 0.6V, CDRV = 10pF, TA = -40°C to +85°C,
unless otherwise noted.) (Note 5)
PARAMETER
CONDITIONS
EN
Input Threshold
Pullup Current
VIL
VIH
V EN = 2.6V
CURRENT LIMIT
VILIM Range
CS Current Limit to DRV Delay
Includes leading-edge blanking time
VCS Cycle-by-Cycle Threshold Voltage
Accuracy when VILIM = 0.075V
VILIM = 1.250V
ILIM Leakage Current
Hiccup Skip Off-Charging Current
Hiccup Skip On-Charging Current
Hiccup SKTOFF Voltage Threshold
DUTY CYCLE
Duty-Cycle Range
VUV = 1.3V
Maximum Duty Cycle
Internal programmed max duty at all frequencies
SOFT-START
Source Current
Pulldown Resistance in Shutdown
FREQUENCY
VSS = 1V
ISS = 10mA, V EN = 3V
Initial Accuracy
RFREQ = 32.4kΩ
SYNC (EXTERNAL FREQUENCY SYNCHRONIZATION)
MIN
2.6
5
0.075
1.130
-1
0.9
1.5
2.3
15
79
4.5
280
TYP
±10
MAX UNITS
1.2
V
µA
1.25
150
1.375
+1
2.1
3.5
2.7
V
ns
%
V
µA
µA
µA
V
80 %
82 %
6.5 µA
30 Ω
320 kHz
Frequency Range
Frequency syncs ±30% of frequency set by
external R on FREQ pin
0.2
1.0 M H z
FREQ Input Threshold
(Note 4)
2 3V
Note 1: This is the VCC operating range after clearing the rising UVLO.
Note 2: Guaranteed by design.
Note 3: See the SKTON and SKTOFF section.
Note 4: The minimum FREQ pulse must be 200ns in duration with a minimum magnitude of 3V plus VF of the external diode and
have a maximum duty cycle of 55%.
Note 5: Specifications to -40°C are guaranteed by design and not production tested.
_______________________________________________________________________________________ 5
5 Page 
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
where RPRAMP is in kΩ. The ramp-voltage magnitude is
independent of frequency. The range of values for
RPRAMP is from 12.4kΩ to 42kΩ.
Soft-Start
The soft-start feature allows converters built using the
MAX8540/MAX8541 to apply power to the load in a
controllable soft ramp, thus reducing startup surges
and stresses. It also determines power-up sequencing
when several converters are used.
Upon power turn-on, the soft-start pin acts as a current
sink to discharge any capacitance connected to it. Once
the voltage at VCC has exceeded its lockout value, soft-
start then charges the external capacitor (CSS), allowing
the converter output voltage to ramp up. Full output volt-
age is reached in approximately 440ms/µF.
Current Limit
The MAX8540/MAX8541 utilize two current-limit
schemes: cycle-by-cycle current limit and short-circuit
current limit. Set the current-limit threshold using a
resistor-divider from REF to GND with ILIM connected to
the center. The current-limit threshold is determined as:
VILIM
=
R5
R5 + R6
×
VREF
where R5 is the resistor from ILIM to GND and R6 is the
resistor from REF to ILIM. Use 240kΩ for R16 and vary
R23 to change the threshold.
The CS signal provides feedback on the current ramp
through the main external MOSFET. The voltage on CS
is monitored by the IC. The cycle-by-cycle current limit
abbreviates the on-time of the external MOSFET in the
event that the voltage at CS is greater than the thresh-
old voltage set by ILIM.
The current-limit feature protects against a hard short
or overcurrent fault at the output by one of two selected
protection modes: by latching off the output, or pulsing
the output to reduce the average output current
(hiccup mode).
To select latched mode, connect SKTOFF to REF. In
this mode, if the hard short exists for the time period set
by the capacitance at SKTON, the output is latched off.
To unlatch the output, toggle EN or cycle the input
power to VCC.
To select hiccup mode, connect capacitors to SKTON
and SKTOFF to program the hiccup mode on- and off-
times. When a cycle-by-cycle event is detected, the IC
charges the capacitor at SKTON. The capacitor contin-
ues to charge as long as the CS voltage is greater than
the ILIM threshold voltage. Once the voltage on SKTON
reaches its threshold voltage, the MAX8540/MAX8541
begin skipping switching cycles for a time determined
by the capacitance connected to SKTOFF. Once this
time period has elapsed, the IC begins to switch for the
time period set by the capacitance connected to
SKTON. This process continues until the output short is
removed. See the SKTON and SKTOFF section for
details on setting the hiccup-mode periods.
Connect SKTOFF to REF and SKTON to GND to disable
the latched-mode and hiccup-mode protection, and
operate continuously in cycle-by-cycle current-limit.
PMW Comparator
The PWM comparator of the MAX8540 transforms the
optocoupled error voltage (VOPTO) into a duty cycle by
comparing the opto feedback-error voltage with a
summed voltage. The summed voltage is the sum of
the programmable slope compensation and the cur-
rent-sense voltages. When the summed voltage
exceeds the opto feedback error voltage, the gate-
drive logic turns off the external MOSFET.
The PWM comparator in the MAX8541 compares
VOPTO with the programmable voltage ramp. When the
voltage ramp exceeds VOPTO, the gate-driver logic
turns off the external MOSFET.
SKTON and SKTOFF
The capacitance, CSKTON, determines the time period
allowed before the short-circuit current limit initiates.
Once the CS voltage exceeds the ILIM threshold, the
capacitor at SKTON begins to charge. The capacitor
continues to charge until the SKTON threshold voltage
is reached or the overcurrent event is removed. This
feature allows for the higher currents required during
startup with high-capacitance loads. Set CSKTON to
allow sufficient time for startup. The required capaci-
tance at SKTON is determined as:
CSKTON = tON / 103
where tON is in ms and CSKTON is in µF. The allowable
range for CSKTON is 100pF to 0.01µF.
The capacitance at SKTOFF determines the time period
that the external MOSFET is turned off during an over-
current event. Once the SKTON time period is exceeded,
the SKTOFF capacitor charges. Once VSKTOFF reaches
its threshold, the IC begins to switch again. CSKTOFF is
determined as:
CSKTOFF = tOFF / 103
where tOFF is in ms and CSKTOFF is in µF. The allow-
able range for CSKTOFF is 1000pF to 1µF.
______________________________________________________________________________________________________ 11
11 Page | ||
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| PDF Descargar | [ Datasheet MAX8541EEE.PDF ] | |
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