LM2597HVM-12 National Semiconductor, LM2597HVM-12 Datasheet - Page 19

• Manufacturer Part Number: LM2597HVM-12
• Description: SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

Typical Circuit and Layout
Guidelines
As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance can
generate voltage transients which can cause problems. For
minimal inductance and ground loops, the wires indicated by
heavy lines should be wide printed circuit traces and
should be kept as short as possible. For best results, ex-
ternal components should be located as close to the
switcher lC as possible using ground plane construction or
single point grounding.
If open core inductors are used, special care must be
taken as to the location and positioning of this type of induc-
tor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and C
When using the adjustable version, special care must be
taken as to the location of the feedback resistors and the as-
sociated wiring. Physically locate both resistors near the IC,
and route the wiring away from the inductor, especially an
open core type of inductor. (See application section for more
information.)
Application Information
PIN FUNCTIONS
+V
switching regulator. A suitable input bypass capacitor must
be present at this pin to minimize voltage transients and to
supply the switching currents needed by the regulator.
Ground (Pin 6) — Circuit ground.
Output (Pin 8) — Internal switch. The voltage at this pin
switches between (+V
with a duty cycle of V
tive circuitry, the PC board copper area connected to this pin
should be kept to a minimum.
Feedback (Pin 4) — Senses the regulated output voltage to
complete the feedback loop.
Shutdown /Soft-start (Pin 5) — This dual function pin pro-
vides the following features: (a) Allows the switching regula-
tor circuit to be shut down using logic level signals thus drop-
ping the total input supply current to approximately 80 µA.
(b) Adding a capacitor to this pin provides a soft-start feature
which minimizes startup current and provides a controlled
ramp up of the output voltage.
Error Flag (Pin 1) — Open collector output that provides a
low signal (flag transistor ON) when the regulated output
voltage drops more than 5% from the nominal output volt-
age. On start up, Error Flag is low until V
the nominal output voltage and a delay time determined by
the Delay pin capacitor. This signal can be used as a reset to
a microprocessor on power-up.
Delay (Pin 2) — At power-up, this pin can be used to provide
a time delay between the time the regulated output voltage
reaches 95% of the nominal output voltage, and the time the
error flag output goes high.
IN
(Pin 7) — This is the positive input supply for the IC
(Continued)
OUT
OUT
IN
/V
− V
wiring can cause problems.
IN
. To minimize coupling to sensi-
SAT
) and approximately −0.5V,
OUT
reaches 95% of
19
Bias Supply (Pin 3) — This feature allows the regulators in-
ternal circuitry to be powered from the regulated output volt-
age or an external supply, instead of the input voltage. This
results in increased efficiency under some operating condi-
tions, such as low output current and/or high input voltage.
Special Note If any of the above four features (Shutdown
/Soft-start, Error Flag, Delay, or Bias Supply) are not used,
the respective pins should be left open.
EXTERNAL COMPONENTS
SOFT-START CAPACITOR
C
Soft-start feature (slow start-up). When the DC input voltage
is first applied to the regulator, or when the Shutdown
/Soft-start pin is allowed to go high, a constant current (ap-
proximately 5 µA begins charging this capacitor). As the ca-
pacitor voltage rises, the regulator goes through four operat-
ing regions (See the bottom curve in Figure 13 ).
1. Regulator in Shutdown. When the SD /SS pin voltage is
between 0V and 1.3V, the regulator is in shutdown, the out-
put voltage is zero, and the IC quiescent current is approxi-
mately 85 µA.
2. Regulator ON, but the output voltage is zero. With the
SD /SS pin voltage between approximately 1.3V and 1.8V,
the internal regulatory circuitry is operating, the quiescent
current rises to approximately 5 mA, but the output voltage is
still zero. Also, as the 1.3V threshold is exceeded, the
Soft-start capacitor charging current decreases from 5 µA
down to approximately 1.6 µA. This decreases the slope of
capacitor voltage ramp.
3. Soft-start Region. When the SD /SS pin voltage is be-
tween 1.8V and 2.8V (
condition. The switch (Pin 8) duty cycle initially starts out
very low, with narrow pulses and gradually get wider as the
capacitor SD /SS pin ramps up towards 2.8V. As the duty
cycle increases, the output voltage also increases at a con-
trolled ramp up. See the center curve in Figure 13 . The input
supply current requirement also starts out at a low level for
the narrow pulses and ramp up in a controlled manner. This
is a very useful feature in some switcher topologies that re-
quire large startup currents (such as the inverting configura-
tion) which can load down the input power supply.
Note: The lower curve shown in Figure 13 shows the Soft-start region from
4. Normal operation. Above 2.8V, the circuit operates as a
standard Pulse Width Modulated switching regulator. The
capacitor will continue to charge up until it reaches the inter-
nal clamp voltage of approximately 7V. If this pin is driven
from a voltage source, the current must be limited to about
1 mA.
SS
— A capacitor on this pin provides the regulator with a
0% to 100%. This is not the duty cycle percentage, but the output volt-
age percentage. Also, the Soft-start voltage range has a negative tem-
perature coefficient associated with it. See the Soft-start curve in the
electrical characteristics section.
@
25˚C), the regulator is in a Soft-start
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