Block Diagram
DS012593-29
FIGURE 12.
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.
Application Information
PIN FUNCTIONS
+VIN (Pin 2)—This is the positive input supply for the IC
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.
EXTERNAL COMPONENTS
SOFT-START CAPACITOR
CSS —A capacitor on this pin provides the regulator with a
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
(approximately 5 µA begins charging this capacitor). As the
capacitor voltage rises, the regulator goes through four op-
erating regions (See the bottom curve in Figure 13).
Ground (Pin 4)—Circuit ground.
Output (Pin 1)—Internal switch. The voltage at this pin
switches between approximately (+VIN − VSAT) and approxi-
mately −0.5V, with a duty cycle of VOUT/VIN. To minimize
coupling to sensitive circuitry, the PC board copper area
connected to this pin should be kept to a minimum.
Feedback (Pin 6)—Senses the regulated output voltage to
complete the feedback loop.
1. Regulator in Shutdown. When the SD /SS pin voltage is
between 0V and 1.3V, the regulator is in shutdown, the
output voltage is zero, and the IC quiescent current is ap-
proximately 85 µA.
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
dropping 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 con-
trolled ramp up of the output voltage.
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
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.
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 VOUT reaches 95% of
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.
3. Soft-start Region. When the SD /SS pin voltage is be-
@
tween 1.8V and 2.8V ( 25˚C), the regulator is in a Soft-start
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
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.
19
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