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MAX15000

Current-Mode PWM Controllers with Programmable Switching Frequency

Maxim Integrated 

Current-Mode PWM Controllers with

Programmable Switching Frequency

n-Channel MOSFET Switch Driver

The NDRV output drives an external n-channel MOSFET.

The internal regulator output (VCC), set to approximately

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

switching frequency (fSW) at the NDRV output:

R12 = 1010 for the MAX15000A /MAX15001A.

2fSW

R12 = 1010 for the MAX15000B /MAX15001B.

4fSW

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:

1

R15 = 88.9 × (R12) 4

For example, if R12 is 4kΩ, R15 becomes 707Ω.

MAX15000 fig04

VOUT

2V/div

100mA LOAD ON/VOUT1

100mA LOAD ON/VOUT2

2ms/div

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

Startup for the Circuit in Figure 6

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:

VOUT

=

1+

R13

R14





VREF

where VREF = 1.23V. The amplifier’s noninverting input

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:

VOUT

=

NS

NT

1+

R1

R2 

VREF



+ VD6  − VD2



where NS is the number of secondary winding turns, NT

is the number of tertiary winding turns, and both VD6

and VD2 are the diode drops at the respective outputs.

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