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MAX5019

Current-Mode PWM Controllers with Integrated Startup Circuit

Maxim Integrated 

Current-Mode PWM Controllers with Integrated

Startup Circuit

Pin Description

PIN

NAME

FUNCTION

1

V+

High-Voltage Startup Input. Connect directly to an input voltage between 18V to 110V. Connects

internally to a high-voltage linear regulator that generates VCC during startup.

VDD is the Input of the Linear Regulator that Generates VCC. For supply voltages less than 36V, VDD

2

VDD

and V+ can both be connected to the supply. For supply voltages greater than 36V, VDD receives

its power from the tertiary winding of the transformer and accepts voltages from 13V to 36V. Bypass

to GND with a 4.7µF capacitor.

3

FB

Input of the Fixed-Gain Inverting Amplifier. Connect a voltage-divider from the regulated output to

this pin. The noninverting input of the amplifier is referenced to 2.4V.

Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 0.45ms/nF.

4

SS_SHDN This pin is also the reference voltage output. Bypass with a minimum 10nF capacitor to GND. The

device goes into shutdown when SS_SHDN is pulled below 0.25V.

Current Sense Input. Turns power switch off if VCS rises above 465mV for cycle-by-cycle current

5

CS

limiting. CS is also the feedback for the current-mode controller. CS is connected to the PWM

comparator through a leading-edge blanking circuit.

6

GND

Ground

7

NDRV

Gate Drive. Drives a high-voltage external N-channel power MOSFET.

Regulated IC Supply. Provides power for the entire IC. VCC is regulated from VDD during normal

8

VCC

operation and from V+ during startup. Bypass VCC with a 10µF tantalum capacitor in parallel with

0.1µF ceramic capacitor to GND.

Detailed Description

Use the MAX5019/MAX5020 PWM current-mode con-

trollers to design flyback- or forward-mode power sup-

plies. Current-mode operation simplifies control-loop

design while enhancing loop stability. An internal high-

voltage startup regulator allows the device to connect

directly to the input supply without an external startup

resistor. Current from the internal regulator starts the

controller. Once the tertiary winding voltage is estab-

lished the internal regulator is switched off and bias

current for running the IC is derived from the tertiary

winding. The internal oscillator is set to 275kHz and

trimmed to ±10%. This permits the use of small mag-

netic components to minimize board space. Both the

MAX5019 and MAX5020 can be used in power sup-

plies providing multiple output voltages. A functional

diagram of the IC is shown in Figure 1. Typical applica-

tions circuits for forward and flyback topologies are

shown in Figure 2 and Figure 3, respectively. For isolat-

ed flyback power supplies use the circuit of Figure 4.

Current-Mode Control

The MAX5019/MAX5020 offer current-mode control

operation with added features such as leading-edge

blanking with dual internal path that only blanks the

sensed current signal applied to the input of the PWM

comparator. The current limit comparator monitors the

CS pin at all times and provides cycle-by-cycle current

limit without being blanked. The leading-edge blanking

of the CS signal prevents the PWM comparator from

prematurely terminating the on cycle. The CS signal

contains a leading-edge spike that is the result of the

MOSFET gate charge current, capacitive and diode

reverse recovery current of the power circuit. Since this

leading-edge spike is normally lower than the current

limit comparator threshold, current limiting is not

blanked and cycle-by-cycle current limiting is provided

under all conditions.

Use the MAX5019 in discontinuous flyback applications

where wide line voltage and load current variation is

expected. Use the MAX5020 for single transistor for-

ward converters where the maximum duty cycle must

be limited to less than 50%.

Under certain conditions it may be advantageous to

use a forward converter with greater than 50% duty

cycle. For those cases use the MAX5019. The large

duty cycle results in much lower operating primary

RMS currents through the MOSFET switch and in most

cases a smaller output filter inductor. The major disad-

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