lm5000sdx-6ep National Semiconductor Corporation, lm5000sdx-6ep Datasheet - Page 12

• Manufacturer Part Number: lm5000sdx-6ep
• Description: Enhanced Plastic High Voltage Switch Mode Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation
The LM5000EP is a current mode PWM regulator. The signal
flow of this control scheme has two feedback loops, one that
senses switch current and one that senses output voltage.
To keep a current programmed control converter stable
above duty cycles of 50%, the inductor must meet certain
criteria. The inductor, along with input and output voltage,
will determine the slope of the current through the inductor
(see Figure 4 (a)). If the slope of the inductor current is too
great, the circuit will be unstable above duty cycles of 50%.
The LM5000EP provides a compensation pin (COMP) to
customize the voltage loop feedback. It is recommended that
a series combination of R
sation network, as shown in Figure 1 . The series combina-
tion of R
following equations:
where R
850kΩ. For most applications, performance can be opti-
mized by choosing values within the range 5kΩ ≤ R
and 680pF ≤ C
COMPENSATION
This section will present a general design procedure to help
insure a stable and operational circuit. The designs in this
datasheet are optimized for particular requirements. If differ-
ent conversions are required, some of the components may
need to be changed to ensure stability. Below is a set of
general guidelines in designing a stable circuit for continu-
ous conduction operation (loads greater than 100mA), in
most all cases this will provide for stability during discontinu-
ous operation as well. The power components and their
effects will be determined first, then the compensation com-
ponents will be chosen to produce stability.
INDUCTOR SELECTION
To ensure stability at duty cycles above 50%, the inductor
must have some minimum value determined by the mini-
mum input voltage and the maximum output voltage. This
equation is:
where fs is the switching frequency, D is the duty cycle, and
R
equation is only good for duty cycles greater than 50%
(D
The inductor ripple current is important for a few reasons.
One reason is because the peak switch current will be the
DSON
>
0.5).
is the ON resistance of the internal switch. This
C
O
and C
is the output impedance of the error amplifier,
C
C
≤ 4.7nF.
introduces pole-zero pair according to the
(Continued)
C
and C
C
be used for the compen-
C
≤ 20kΩ
12
average inductor current (input current) plus ∆i
be taken to make sure that the switch will not reach its
current limit during normal operation. The inductor must also
be sized accordingly. It should have a saturation current
rating higher than the peak inductor current expected. The
output voltage ripple is also affected by the total ripple cur-
rent.
DC GAIN AND OPEN-LOOP GAIN
Since the control stage of the converter forms a complete
feedback loop with the power components, it forms a closed-
loop system that must be stabilized to avoid positive feed-
back and instability. A value for open-loop DC gain will be
required, from which you can calculate, or place, poles and
zeros to determine the crossover frequency and the phase
margin. A high phase margin (greater than 45˚) is desired for
the best stability and transient response. For the purpose of
stabilizing the LM5000EP, choosing a crossover point well
below where the right half plane zero is located will ensure
sufficient phase margin. A discussion of the right half plane
zero and checking the crossover using the DC gain will
follow.
OUTPUT CAPACITOR SELECTION
The choice of output capacitors is somewhat more arbitrary.
It is recommended that low ESR (Equivalent Series Resis-
tance, denoted R
polymer electrolytic, or low ESR tantalum. Higher ESR ca-
pacitors may be used but will require more compensation
which will be explained later on in the section. The ESR is
also important because it determines the output voltage
ripple according to the approximate equation:
After choosing the output capacitor you can determine a
pole-zero pair introduced into the control loop by the follow-
ing equations:
Where R
the maximum load current. The zero created by the ESR of
the output capacitor is generally very high frequency if the
ESR is small. If low ESR capacitors are used it can be
neglected. If higher ESR capacitors are used see the High
Output Capacitor ESR Compensation section.
RIGHT HALF PLANE ZERO
A current mode control boost regulator has an inherent right
half plane zero (RHP zero). This zero has the effect of a zero
in the gain plot, causing an imposed +20dB/decade on the
rolloff, but has the effect of a pole in the phase, subtracting
another 90˚ in the phase plot. This can cause undesirable
effects if the control loop is influenced by this zero. To ensure
the RHP zero does not cause instability issues, the control
loop should be designed to have a bandwidth of
frequency of the RHP zero or less. This zero occurs at a
frequency of:
L
is the minimum load resistance corresponding to
∆V
ESR
OUT
) capacitors be used such as ceramic,
) 2∆i
L
R
ESR
(in Volts)
L
. Care must
1
⁄
2
the