MAX8541 MAXIM [Maxim Integrated Products], MAX8541 Datasheet - Page 10

• Manufacturer Part Number: MAX8541
• Description: Synchronizable, High-Frequency Current- and Voltage-Mode PWM Controllers for Isolated Supplies
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX8540/MAX8541 also synchronize with an
external oscillator. Drive FREQ/SYNC with a square
wave through a series diode with a positive pulse width
of at least 200ns and a minimum pulse amplitude of 3V,
plus the V
level-shifted external clock can be applied without the
series diode. The maximum duty cycle of the external
signal allowed is 55%. The MAX8540/MAX8541 syn-
chronize to frequencies between 200kHz and 1MHz;
however, the signal must be within ±30% of the fre-
quency set by the external resistor at FREQ/SYNC.
Set the maximum duty cycle at the minimum system
input voltage (V
MAXDTY to GND. The maximum duty cycle is inversely
proportional to the voltage at UV. As the voltage on UV
increases, the duty cycle decreases. The maximum
duty cycle is internally limited to 80% at all switching
frequencies. The MAXDTY resistor is determined as:
where D
The range of valid resistor values for R
24.3kΩ to 130kΩ.
The DRV output drives an N-channel MOSFET in low-
power applications. In high-power applications, the gate
driver internal to the MAX8540/MAX8541 may not be
capable of driving the external MOSFET efficiently and
an external gate driver may be required. In this situation,
connect DRV to the input of the external gate driver.
The MAX8540 is a current-mode device and requires
slope compensation for proper operation. To provide
slope compensation, connect a resistor from SCOMP to
GND (R
follows.
For applications using a synchronous rectifier in the
output, set the slope compensation equal to the nega-
tive slope of the output inductor. R
Synchronizable, High-Frequency Current- and
Voltage-Mode PWM Controllers for Isolated Supplies
10
______________________________________________________________________________________
SCOMP
MAX
F
R
of the external diode. Alternatively, a 1.5V
is the desired maximum duty cycle.
MAXDTY
R
Slope Compensation (MAX8540)
). The value of R
SCOMP
IN(MIN)
SCF
N-Channel MOSFET Driver
=
=
=
dV
D
) connecting a resistor from
N
MAX
RAMP
dt
V
Maximum Duty Cycle
OUT
L
1
SCOMP
×
×
R
(
60
SCOMP
97 6
CS
R
2
.
SCF
SUM
is determined as
×
MAXDTY
10
is equal to:
3
)
is from
where dV
turns ratio of the primary to secondary, L1 is the output
inductance, V
ing frequency, and R
For applications where a diode is used in the output
instead of the synchronous rectifier, the slope compen-
sation resistor is then equal to:
where dV
ratio of the primary to secondary, L1 is the output induc-
tance, V
f
sense resistance.
The MAX8541 is a voltage-mode device and features
adjustable voltage ramp. Connect a resistor from
PRAMP to GND (R
magnitude, V
Figure 1. A Method to Set UV and OV Thresholds
S
is the switching frequency, and R
36V TO 76V
OUT
Voltage-Ramp Amplitude (MAX8541)
RAMP
V
RAMP
IN
R
is the output voltage, V
SCF
SCOMP
M
OUT
/dt = 2.5V(f
.
/dt = 2.5V(f
V
M
C1
RC
=
is the output voltage, f
= 1 25
RA
RB
N
=
PRAMP
CS
dV
.
V
C2
is the current-sense resistance.
OUT
RAMP
dt
S
V
),R
L
S
) to set the voltage-ramp
1
R
+
), R
SUM
PRAMP
25
C3
OV
UV
V
×
F
k
SUM
Ω
R
2
R
= 25kΩ, N is the turns
F
MAX8541
SCF
SUM
CS
is the diode voltage,
CS
= 25kΩ, N is the
S
is the current-
is the switch-