Detailed Description

Overvoltage Threshold

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):

age trip point, R1 is the resistor from the system input

to OV, and R2 is the resistor from OV to GND.

Undervoltage Threshold

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.

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 36.5k

Ω for the bottom resistor (RC). RA

and RB are calculated as follows:

is the OV threshold (3.021V typ). RA should consist of

two equal-value resistors in series to protect against

single-point failure.

Switching Frequency and Synchronization

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.

RA

RC

V

V

RB

RC

IN MIN

UV

=

×

−−

()

RB

RC

VV

VV

OV

IN MIN

UV

IN MAX

=×

×

×













−

()

()

1

V

RR

R

V

IN MIN

UV

+









×

34

4

V

RR

R

V

IN MAX

OV

+









×

12

2

Synchronizable, High-Frequency Current- and

Voltage-Mode PWM Controllers for Isolated Supplies

_______________________________________________________________________________________

9

Table 1. Typical Specifications of UV/OV

and the Actual On/Off Hysteresis of Input

Voltage

UV VOLTAGE

SPECIFICATIONS

(V)

WINDOW (LTP)

(V)

WINDOW (UTP)

(V)

MIN

1.083

29.76

TYP

1.128

31.00

MAX

1.173

32.24

—

MIN

1.200

32.97

TYP

1.250

34.34

MAX

1.300

—

36.00

OV VOLTAGE

SPECIFICATIONS

(V)

WINDOW (UTP)

(V)

WINDOW (LTP)

(V)

MIN

2.901

79.70

TYP

3.021

83.00

MAX

3.142

86.32

—

MIN

2.778

76.32

TYP

2.894

—

79.50

MAX

3.010

82.68

Assume that 1.250V at UV/OV is scaled to 34.34V for a 48V bus.

Ω)

SWITCHING FREQUENCY (kHz)

48.7

200

19.1

500

15.8

600

11.0

800

8.87

1000

Table 2. Switching Frequency Selection