n-Channel MOSFET Switch Driver

The NDRV output drives an external n-channel MOSFET.

9V, drives NDRV. For the universal input voltage range,

the MOSFET used must withstand the DC level of the

high-line input voltage plus the reflected voltage at the

primary of the transformer. Most applications that use the

discontinuous flyback topology require a MOSFET rated

at 600V. NDRV can source/sink in excess of 650/1000mA

peak current; therefore, select a MOSFET that will yield

acceptable conduction and switching losses.

Oscillator/Switching Frequency

Use an external resistor at RT to program the

MAX15000/MAX15001 internal oscillator frequency

between 50kHz and 2.5MHz. The MAX15000A/

MAX15001A output switching frequency is one-half of

the programmed oscillator frequency with a 50% duty

cycle. The MAX15000B/MAX15001B output switching

frequency is one-quarter of the programmed oscillator

frequency with a 75% duty cycle.

The MAX15000A/MAX15001A and MAX15000B/

MAX15001B have programmable output switching fre-

quencies from 25kHz to 625kHz and 12.5kHz to

625kHz, respectively. Use the following formulas to

determine the appropriate value of the resistor R12

(see Figure 1) needed to generate the desired output

where R12 is the resistor connected from RT to GND

(see Figure 1).

Connect an RC network in parallel with R12 as shown in

Figure 1. The RC network should consist of a 100nF

capacitor C6 (for stability) in series with resistor R15

which serves to further minimize jitter. Use the following

formula to determine the value of R15:

For example, if R12 is 4k

Ω, R15 becomes 707Ω.

Internal Error Amplifier

The MAX15000/MAX15001 include an internal error

amplifier to regulate the output voltage in the case of a

nonisolated power supply (see Figure 1). For the circuit

in Figure 1, calculate the output voltage using the fol-

lowing equation:

is internally connected to a digital soft-start circuit that

gradually increases the reference voltage during start-

up applied to this input. This forces the output voltage

to come up in an orderly and well-defined manner

under all load conditions.

The error amplifier may also be used to regulate the ter-

tiary winding output which implements a primary-side-

regulated, isolated power supply (see Figure 6). For the

circuit in Figure 6, calculate the output voltage using

the following equation:

V

N

N

R

R

VV

V

OUT

S

T

REF

D

D

=+









+











 −

1

1

2

62

V

R

R

V

OUT

REF

=+









1

13

14

RR

15

88 9

12

1

4

.

=×

()

R

f

for the MAX

A MAX

A

R

f

for the MAX

B MAX

B

SW

SW

12

10

2

15000

15001

12

10

4

15000

15001

10

10

=

=

/.

/.

Current-Mode PWM Controllers with

Programmable Switching Frequency

______________________________________________________________________________________

11

MAX15000 fig04

2ms/div

2V/div

Figure 4. Primary-Side Output Voltage Soft-Start During Initial

Startup for the Circuit in Figure 6