MAX8540EEE+ Maxim Integrated Products, MAX8540EEE+ Datasheet - Page 11

• Manufacturer Part Number: MAX8540EEE+
• Description: IC PWM CTRLR CURR-MODE 16-QSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX8540EEE+

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 82%
• Voltage - Supply: 7.6 V ~ 18 V
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: Yes
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 16-QSOP
• Frequency-max: 1MHz
• Duty Cycle (max): 80 %
• Mounting Style: SMD/SMT
• Switching Frequency: 200 KHz to 1000 KHz
• Maximum Operating Temperature: + 85 C
• Fall Time: 53 ns
• Minimum Operating Temperature: - 40 C
• Rise Time: 57 ns
• Synchronous Pin: Yes
• Topology: Flyback, Forward
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

where R
independent of frequency. The range of values for
R
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 V
start then charges the external capacitor (C
the converter output voltage to ramp up. Full output volt-
age is reached in approximately 440ms/µF.
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:
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 V
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
______________________________________________________________________________________________________
Voltage-Mode PWM Controllers for Isolated Supplies
PRAMP
PRAMP
is from 12.4kΩ to 42kΩ.
CC
.
V
is in kΩ. The ramp-voltage magnitude is
CC
ILIM
Synchronizable, High-Frequency Current- and
has exceeded its lockout value, soft-
=
R
5
R
+
5
R
6
×
V
REF
Current Limit
Soft-Start
SS
), allowing
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.
The PWM comparator of the MAX8540 transforms the
optocoupled error voltage (V
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
V
voltage ramp exceeds V
turns off the external MOSFET.
The capacitance, C
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 C
allow sufficient time for startup. The required capaci-
tance at SKTON is determined as:
where t
range for C
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 V
its threshold, the IC begins to switch again. C
determined as:
where t
able range for C
OPTO
with the programmable voltage ramp. When the
ON
OFF
is in ms and C
SKTON
is in ms and C
SKTOFF
C
C
is 100pF to 0.01µF.
SKTOFF
SKTON
SKTON
is 1000pF to 1µF.
SKTON
OPTO
, determines the time period
= t
= t
SKTOFF
OPTO
SKTON and SKTOFF
ON
OFF
, the gate-driver logic
/ 10
is in µF. The allowable
PMW Comparator
/ 10
) into a duty cycle by
is in µF. The allow-
3
3
SKTOFF
SKTON
SKTOFF
reaches
11
to
is