capacitor (5µF to 10µF) at the V
CC
pin, since the rail will
not support such a load. It is this current, equivalent to
the product of the total gate switching charge (from the
N-channel MOSFET data sheet), times the operating
frequency, that determines the bulk of the MAX5003
power dissipation.
The driver can source up to 560mA and sink up to 1A
transient current with a typical on source resistance of
4Ω. The no-load output levels are V
CC
and PGND.
Applications Information
Compensation and Loop
Design Considerations
The circuit shown in Figure 2 is essentially an energy
pump. It stores energy in the magnetic core and the air
gap of the transformer while the power switch is on,
and delivers it to the load during the off phase. It can
operate in two modes: continuous and discontinuous.
In discontinuous mode, all the energy is given to the
load before the next cycle begins; in continuous mode,
some energy is continuously stored in the core.
The system has four operating parameters: input volt-
age, output voltage, load current, and duty cycle. The
PWM controller senses the output voltage and the input
voltage, and keeps the output voltage regulated by
controlling the duty cycle.
The output filter in this circuit consists of the load resis-
tance and the capacitance on the output.
To study the stability of the feedback system and
design the compensation necessary for system stability
under all operating conditions, first determine the trans-
fer function. In discontinuous mode, since there is no
energy stored in the inductor at the end of the cycle,
the inductor and capacitor do not show the characteris-
tic double pole, and there is only a dominant pole
defined by the filter capacitor and the load resistance.
There is a zero at a higher frequency, defined by the
ESR of the output filter capacitor. Such a response is
easy to stabilize for a wide range of operating condi-
tions while retaining a reasonably fast loop response.
In continuous mode, the situation is different. The
inductor-capacitor combination creates a double pole,
since energy is stored in the inductor at all times. In
addition to the double pole, a right-half-plane zero
appears in the frequency response curves. This
response is not easy to compensate. It can result in
conditional stability, a complicated compensation net-
work, or very slow transient response.
To avoid the analytical and design problems of the con-
tinuous-conduction mode flyback topology and maintain
good loop response, choose a design incorporating a
discontinuous-conduction mode power stage
To keep the converter in discontinuous mode at all times,
the value of the power transformer’s primary inductance
must be calculated at minimum line voltage and maxi-
mum load, and the maximum duty cycle must be limited.
The MAX5003 has a programmable duty-cycle limit func-
tion intended for this purpose.
Design Methodology
Following is a general procedure for developing a sys-
tem:
1) Determine the requirements.
2) In free-running mode, choose the FREQ pin pro-
gramming resistor. In synchronized mode, determine
the clock frequency (f
CLK)
.
3) Determine the transformer turns ratio, and check the
maximum duty cycle.
4) Determine the transformer primary inductance.
5) Complete the transformer specifications by listing
the primary maximum current, the secondary maxi-
mum current, and the minimum duty cycle at full
power.
6) Choose the MAXTON pin programming resistor.
7) Choose a filter capacitor.
8) Determine the compensation network.
Design Example
1) 36V < V
IN
< 72V, V
OUT
= 5V, I
OUT
= 1A, ripple
< 50mV, settling time ≈ 0.5ms.
2) Generally, the higher the frequency, the smaller the
transformer. A higher frequency also gives higher
system bandwidth and faster settling time. The
trade-off is lower efficiency. In this example, 300kHz
switching frequency is the choice to favor for a small
transformer. If the converter will be free running (not
externally synchronized), use the following formula to
calculate the R
FREQ
programming resistor:
where:
R
FREQ
= Resistor between FREQ and ground
ƒ
SW
= Switching frequency (300kHz)
If the converter is synchronized to an external clock,
the input frequency will be 1.2MHz. The external
clock runs at four times the desired switching fre-
quency.
