LM2598S-ADJ National Semiconductor, LM2598S-ADJ Datasheet - Page 21

• Manufacturer Part Number: LM2598S-ADJ
• Description: SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

Application Information
DELAY CAPACITOR
C
upper curve in Figure 13 , and also refer to timing diagrams in
Figure 14 . A capacitor on this pin provides a time delay be-
tween the time the regulated output voltage (when it is in-
creasing in value) reaches 95% of the nominal output volt-
age, and the time the error flag output goes high. A 3 µA
constant current from the delay pin charges the delay ca-
pacitor resulting in a voltage ramp. When this voltage
reaches a threshold of approximately 1.3V, the open collec-
tor error flag output (or power OK) goes high. This signal can
be used to indicate that the regulated output has reached the
correct voltage and has stabilized.
If, for any reason, the regulated output voltage drops by 5%
or more, the error output flag (Pin 3) immediately goes low
(internal transistor turns on). The delay capacitor provides
very little delay if the regulated output is dropping out of
regulation. The delay time for an output that is decreasing is
approximately a 1000 times less than the delay for the rising
output. For a 0.1 µF delay capacitor, the delay time would be
approximately 50 ms when the output is rising and passes
through the 95% threshold, but the delay for the output drop-
ping would only be approximately 50 µs.
R
tor of a NPN transistor, with the emitter internally grounded.
To use the error flag, a pullup resistor to a positive voltage is
needed. The error flag transistor is rated up to a maximum of
45V and can sink approximately 3 mA. If the error flag is not
used, it can be left open.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
C
Figure 1 is used when the output voltage is greater than 10V
or then C
compensation to the feedback loop and increases the phase
margin for better loop stability. For C
sign procedure section.
If the output ripple is large (
age), this ripple can be coupled to the feedback pin through
the feedforward capacitor and cause the error comparator to
trigger the error flag. In this situation, adding a resistor, R
in series with the feedforward capacitor, approximately 3
times R1, will attenuate the ripple voltage at the feedback
pin.
INPUT CAPACITOR
C
needed between the input pin and ground pin. It must be lo-
cated near the regulator using short leads. This capacitor
prevents large voltage transients from appearing at the in-
put, and provides the instantaneous current needed each
time the switch turns on.
The important parameters for the Input capacitor are the
voltage rating and the RMS current rating. Because of the
relatively high RMS currents flowing in a buck regulator’s in-
put capacitor, this capacitor should be chosen for its RMS
current rating rather than its capacitance or voltage ratings,
although the capacitance value and voltage rating are di-
rectly related to the RMS current rating.
The RMS current rating of a capacitor could be viewed as a
capacitor’s power rating. The RMS current flowing through
the capacitors internal ESR produces power which causes
DELAY
Pull Up
FF
IN
— A Feedforward Capacitor C
— A low ESR aluminum or tantalum bypass capacitor is
— Provides delay for the error flag output. See the
— The error flag output, (or power OK) is the collec-
OUT
has a very low ESR. This capacitor adds lead
>
5% of the nominal output volt-
FF
FF
, shown across R2 in
selection, see the de-
(Continued)
FF
,
21
the internal temperature of the capacitor to rise. The RMS
current rating of a capacitor is determined by the amount of
current required to raise the internal temperature approxi-
mately 10˚C above an ambient temperature of 105˚C. The
ability of the capacitor to dissipate this heat to the surround-
ing air will determine the amount of current the capacitor can
safely sustain. Capacitors that are physically large and have
a large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage electro-
lytic capacitor will be physically larger than a lower voltage
capacitor, and thus be able to dissipate more heat to the sur-
rounding air, and therefore will have a higher RMS current
rating.
FIGURE 16. Capacitor ESR vs Capacitor Voltage Rating
The consequences of operating an electrolytic capacitor
above the RMS current rating is a shortened operating life.
The higher temperature speeds up the evaporation of the ca-
pacitor’s electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manu-
facturers data sheet for maximum allowable RMS ripple cur-
rent. For a maximum ambient temperature of 40˚C, a gen-
eral guideline would be to select a capacitor with a ripple
current rating of approximately 50% of the DC load current.
For ambient temperatures up to 70˚C, a current rating of
75% of the DC load current would be a good choice for a
conservative design. The capacitor voltage rating must be at
least 1.25 times greater than the maximum input voltage,
and often a much higher voltage capacitor is needed to sat-
isfy the RMS current requirements.
FIGURE 15. RMS Current Ratings for Low
(Typical Low ESR Electrolytic Capacitor)
ESR Electrolytic Capacitors (Typical)
DS012593-33
DS012593-32
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