MAX5019/MAX5020
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-
Current-Mode PWM Controllers with Integrated
Startup Circuit
6 _______________________________________________________________________________________
Pin Description
High-Voltage Startup Input. Connect directly to an input voltage between 18V to 110V. Connects
during startup.
. For supply voltages less than 36V, V
and V+ can both be connected to the supply. For supply voltages greater than 36V, V
its power from the tertiary winding of the transformer and accepts voltages from 13V to 36V. Bypass
to GND with a 4.7µF capacitor.
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.
This pin is also the reference voltage output. Bypass with a minimum 10nF capacitor to GND. The
Current Sense Input. Turns power switch off if V
limiting. CS is also the feedback for the current-mode controller. CS is connected to the PWM
comparator through a leading-edge blanking circuit.
Regulated IC Supply. Provides power for the entire IC. V
operation and from V+ during startup. Bypass V
0.1µF ceramic capacitor to GND.
