Datasheet

LM2599
SNVS123C –APRIL 1998–REVISED APRIL 2013 www.ti.com
APPLICATION INFORMATION
PIN DESCRIPTIONS
+V
IN
(Pin 1)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.
Ground (Pin 4)Circuit ground.
Output (Pin 2)Internal switch. The voltage at this pin switches between approximately (+V
IN
− V
SAT
) and
approximately −0.5V, with a duty cycle of V
OUT
/V
IN
. 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.
Shutdown /Soft-start (Pin 7)This dual function pin provides the following features: (a) Allows the switching
regulator circuit to be shut down using logic level signals thus dropping 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 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 voltage. On start up, Error Flag is low until V
OUT
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.
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.
Special NoteIf 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
SS
— 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 operating regions (See the bottom curve in Figure 29).
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 approximately 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 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 between 1.8V and 2.8V (@ 25°C), the regulator is in a
Soft-start condition. The switch (Pin 2) 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 controlled ramp up. See the center curve in Figure 29. 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 require large startup currents (such as the
inverting configuration) which can load down the input power supply.
Note: The lower curve shown in Figure 29 shows the Soft-start region from 0% to 100%. This is not the duty
cycle percentage, but the output voltage percentage. Also, the Soft-start voltage range has a negative
temperature coefficient associated with it. See the Soft-start curve in the Electrical Characteristics section.
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 internal clamp voltage of approximately
7V. If this pin is driven from a voltage source, the current must be limited to about 1 mA.
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