MAX5019/MAX5020
pass filter (Figures 2, 3). Select the current-sense resis-
tor, R
SENSE
according to the following equation:
where I
LimPrimary
is the maximum peak primary-side
current.
When V
CS
> 465mV, the power MOSFET switches off.
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:
where V
REF
= 2.4V.
Choose R
1
//R
2
<< R
IN
, where R
IN
, ≅ 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:
where N
S
is the number of secondary turns and N
T
is
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:
where I
PRIMARY
is the current through the N-channel
MOSFET, V
REF
is the 2.4V internal reference, V
EA
is the
output voltage of the internal amplifier, and V
SCOMP
is
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:
where k = 0.75 to 1, and N
S
and N
P
are the number of
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-
enced across R
SENSE
equal to the slope compensa-
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
transients, bypass V
CC
with a ceramic capacitor. The
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:
