LM2598S-ADJ National Semiconductor, LM2598S-ADJ Datasheet - Page 19

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

Block Diagram
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 4) — Circuit ground.
Output (Pin 1) — Internal switch. The voltage at this pin
switches between approximately (+V
mately −0.5V, with a duty cycle of V
coupling to sensitive circuitry, the PC board copper area con-
nected to this pin should be kept to a minimum.
Feedback (Pin 6) — Senses the regulated output voltage to
complete the feedback loop.
Shutdown /Soft-start (Pin 7) — 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 85 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 3) — 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 5) — 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 2) — This is the positive input supply for the IC
IN
OUT
− V
OUT
/V
SAT
IN
reaches 95% of
) and approxi-
. To minimize
FIGURE 12.
19
Special Note If any of the above three features (Shutdown
/Soft-start, Error Flag, or Delay) 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 regulator circuitry is operating, the quiescent cur-
rent 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 1) 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
SS
— A capacitor on this pin provides the regulator with a
@
25˚C), the regulator is in a Soft-start
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DS012593-29