MAX8541 MAXIM [Maxim Integrated Products], MAX8541 Datasheet - Page 9

• 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 include an overvoltage protec-
tion (OVP) feature that turns off the external MOSFET
when the input voltage exceeds the user-set threshold.
Connect a resistor-divider from the system input to
GND with OV connected to the center to set the OVP
trip point. The threshold voltage for OV is 3.021V (typ):
where V
age trip point, R1 is the resistor from the system input
to OV, and R2 is the resistor from OV to GND.
The MAX8540/MAX8541 also include an undervoltage
(UV) sensing input. The IC holds the external MOSFET
low until UV reaches its threshold (1.25V typ). Once the
threshold has been reached, the circuit enters soft-start
and brings the output into regulation. Connect a resistor-
divider from the system input to GND with UV at the cen-
ter to set the undervoltage protection (UVP) trip point.
where V
point, R3 is the resistor from the system input to UV,
and R4 is the resistor from UV to GND.
An alternate method used in the application circuits for
setting the OV and UV trip points is demonstrated in
Figure 1. Use 36.5kΩ for the bottom resistor (RC). RA
and RB are calculated as follows:
where V
trip point, V
is the OV threshold (3.021V typ). RA should consist of
two equal-value resistors in series to protect against
single-point failure.
Voltage-Mode PWM Controllers for Isolated Supplies
OV
IN(MIN)
UV
RB
is the OV threshold, V
UV
is the UV threshold, V
V
RA
V
=
IN MAX
IN MIN
(
is the UV threshold (1.25V typ), and V
RC
(
is the UV trip point, V
=
Synchronizable, High-Frequency Current- and
_______________________________________________________________________________________
RC
)
×
)
=



=
Detailed Description



V
×
V



V
R
UV
OV
Undervoltage Threshold
R
V
UV
1
IN MIN
Overvoltage Threshold
3
R
+
R
×
(
2
×
+
R
4
V
V
2
R
IN MAX
IN MIN
)

 ×

4
(
IN(MAX)
(

 ×

−
IN(MIN)
RB
V
OV
)
)
V
IN(MAX)
UV
−
−
RC
is the overvolt-
1
is the UV trip



is the OV
OV
The MAX8540/MAX8541 oscillator operates in two
modes: stand-alone or synchronized (sync). A single
input, FREQ/SYNC, doubles as the attachment point for
the frequency-programming resistor and as the syn-
chronization input. The mode recognition is automatic,
based on the signal applied to FREQ/SYNC.
In stand-alone mode, an external resistor connected
from FREQ/SYNC to GND sets the operating frequency.
A 1.25V source is internally applied to FREQ/SYNC and
the oscillator frequency is proportional to the current
out of FREQ/SYNC through the programming resistor.
Table 1. Typical Specifications of UV/OV
and the Actual On/Off Hysteresis of Input
Voltage
Table 2. Switching Frequency Selection
Assume that 1.250V at UV/OV is scaled to 34.34V for a 48V bus.
Switching Frequency and Synchronization
SPECIFICATIONS
SPECIFICATIONS
MAX
MAX
MAX
MAX
MIN
MIN
MIN
MIN
TYP
TYP
TYP
TYP
R
UV VOLTAGE
OV VOLTAGE
FREQ/SYNC
48.7
19.1
15.8
11.0
8.87
(V)
(V)
1.083
1.128
1.173
1.200
1.250
1.300
2.901
3.021
3.142
2.778
2.894
3.010
(kΩ)
WINDOW (UTP)
WINDOW (LTP)
SWITCHING FREQUENCY (kHz)
V
V
29.76
31.00
32.24
79.70
83.00
86.32
IN
IN
(V)
(V)
—
—
OFF
OFF
1000
200
500
600
800
WINDOW (UTP)
WINDOW (LTP)
V
V
32.97
34.34
36.00
76.32
79.50
82.68
IN
IN
(V)
(V)
—
—
ON
ON
9