Datasheet

LM2591HV
SNVS074D –MAY 2001–REVISED APRIL 2013 www.ti.com
(Adjustable Output Voltage Version)
If the output voltage 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. The voltage rating of the capacitor and its RMS ripple current
capability must never be exceeded.
OUTPUT CAPACITOR
C
OUT
—An output capacitor is required to filter the output and provide regulator loop stability. Low impedance or
low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used.
When selecting an output capacitor, the important capacitor parameters are; the 100 kHz Equivalent Series
Resistance (ESR), the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor,
the ESR value is the most important parameter. The ESR should generally not be less than 100 mΩ or there will
be loop instability. If the ESR is too large, efficiency and output voltage ripple are effected. So ESR must be
chosen carefully.
CATCH DIODE
Buck regulators require a diode to provide a return path for the inductor current when the switch turns off. This
must be a fast diode and must be located close to the LM2591HV using short leads and short printed circuit
traces.
Because of their very fast switching speed and low forward voltage drop, Schottky diodes provide the best
performance, especially in low output voltage applications (5V and lower). Ultra-fast recovery, or High-Efficiency
rectifiers are also a good choice, but some types with an abrupt turnoff characteristic may cause instability or
EMI problems. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. The diode must
be chosen for its average/RMS current rating and maximum voltage rating. The voltage rating of the diode must
be greater than the DC input voltage (not the output voltage).
DELAYED STARTUP
The circuit in Figure 29 uses the ON /OFF pin to provide a time delay between the time the input voltage is
applied and the time the output voltage comes up (only the circuitry pertaining to the delayed start up is shown).
As the input voltage rises, the charging of capacitor C1 pulls the ON /OFF pin high, keeping the regulator off.
Once the input voltage reaches its final value and the capacitor stops charging, and resistor R
2
pulls the ON
/OFF pin low, thus allowing the circuit to start switching. Resistor R
1
is included to limit the maximum voltage
applied to the ON /OFF pin (maximum of 25V), reduces power supply noise sensitivity, and also limits the
capacitor, C1, discharge current. When high input ripple voltage exists, avoid long delay time, because this ripple
can be coupled into the ON /OFF pin and cause problems.
This delayed startup feature is useful in situations where the input power source is limited in the amount of
current it can deliver. It allows the input voltage to rise to a higher voltage before the regulator starts operating.
Buck regulators require less input current at higher input voltages.
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