pass filter (Figures 2, 3). Select the current-sense resis-

current.

The propagation delay from the time the switch current

reaches the trip level to the driver turn-off time is 180ns.

Internal Error Amplifier

The MAX5019/MAX5020 include an internal error ampli-

fier that can be used to regulate the output voltage in

the case of a nonisolated power supply (see Figure 2).

Calculate the output voltage using the following equa-

tion:

≅ 50kΩ is the input

resistance of FB. The gain of the error amplifier is inter-

nally configured for -20 (see Figure 1).

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

put of the tertiary winding for implementing a primary-

side regulated isolated power supply (see Figure 4).

Calculate the output voltage using the following equation:

the number of tertiary winding turns.

PWM Comparator and Slope Compensation

An internal 275kHz oscillator determines the switching

frequency of the controller. At the beginning of each

cycle, NDRV switches the N-channel MOSFET on.

NDRV switches the external MOSFET off after the maxi-

mum duty cycle has been reached, regardless of the

feedback.

The MAX5019 uses an internal ramp generator for

slope compensation. The internal ramp signal is reset

at the beginning of each cycle and slews at 26mV/µs.

The PWM comparator uses the instantaneous current,

the error voltage, the internal reference, and the slope

compensation (MAX5019 only) to determine when to

switch the N-channel MOSFET off. In normal operation

the N-channel MOSFET turns off when:

a ramp function starting at 0 and slewing at 26mV/µs

(MAX5019 only). When using the MAX5019 in a for-

ward-converter configuration the following condition

must be met to avoid control-loop subharmonic oscilla-

tions:

turns on the secondary and primary side of the trans-

former, respectively. L is the output filter inductor. This

makes the output inductor current downslope as refer-

tion. The controller responds to transients within one

cycle when this condition is met.

N-Channel MOSFET Gate Driver

NDRV drives an N-channel MOSFET. NDRV sources

and sinks large transient currents to charge and dis-

charge the MOSFET gate. To support such switching

average current as a result of switching the MOSFET is

the product of the total gate charge and the operating

frequency. It is this current plus the DC quiescent cur-

rent that determines the total operating current.

Applications Information

Design Example

The following is a general procedure for designing a

forward converter using the MAX5020.

1) Determine the requirements.

2) Set the output voltage.

3) Calculate the transformer primary to secondary

winding turns ratio.

4) Calculate the reset to primary winding turns ratio.

5) Calculate the tertiary to primary winding turns

ratio.

6) Calculate the current-sense resistor value.

7) Calculate the output inductor value.

8) Select the output capacitor.

The circuit in Figure 2 was designed as follows:

N

N

kR

V

S

P

SENSE

OUT

× ××

=µ

L

mV

s

26

/

IR

V

- V

- V

PRIMARY

SENSE

EA

REF

SCOMP

×>

V

N

N

R

R

V

OUT

S

T

1

2

REF

=+









×

1

V

R

R

V

OUT

1

2

REF

=+









×

1

RI

SENSE

LimPrimary

= 0 465

./

V

Current-Mode PWM Controllers with Integrated

Startup Circuit

10

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