Datasheet
Z1
3V
C
SS
SD/SS
V
IN
LM2598
5
Q1
LM2598
www.ti.com
SNVS125C –MARCH 1998–REVISED APRIL 2013
Figure 32. External 3.7V Soft-Start Clamp
DELAY CAPACITOR
C
DELAY
— Provides delay for the error flag output. See the upper curve in Figure 30, and also refer to timing
diagrams in Figure 31. A capacitor on this pin provides a time delay between the time the regulated output
voltage (when it is increasing in value) reaches 95% of the nominal output voltage, and the time the error flag
output goes high. A 3 μA constant current from the delay pin charges the delay capacitor resulting in a voltage
ramp. When this voltage reaches a threshold of approximately 1.3V, the open collector 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 dropping would only
be approximately 50 μs.
R
Pull Up
—The error flag output, (or power OK) is the collector 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
FF
— A Feedforward Capacitor C
FF
, shown across R2 in Figure 24 is used when the output voltage is greater
than 10V or then C
OUT
has a very low ESR. This capacitor adds lead compensation to the feedback loop and
increases the phase margin for better loop stability. For C
FF
selection, see the Design Procedure section.
If the output ripple is large (> 5% of the nominal output voltage), 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
FF
, in series with the feedforward capacitor, approximately 3 times R1, will attenuate the
ripple voltage at the feedback pin.
INPUT CAPACITOR
C
IN
— A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. It
must be located near the regulator using short leads. This capacitor prevents large voltage transients from
appearing at the input, 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 input 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 directly related to the RMS current rating.
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