TLE6389 Datasheet, PDF (20/23 Pages) Infineon Technologies AG – Step-Down DC/DC Controller

TLE 6389
4.5
Quiescent Current
The device’s typical quiescent current is 50µA. However, actual applications draw
additional current to supply MOSFET switching currents, FB pin current, or external
feedback resistors (if used), and both the diode and capacitor leakage currents. For
example, with VS at 13.5V and VOUT at 5V, typical no-load supply current for the entire
circuit is TBDµA. When designing a circuit for high-temperature operation, select a
Schottky diode with low reverse leakage.
5
Application Information
5.1
Output Voltage Selection (TLE6389GV only)
Select an output voltage between 5V and 15V by connecting FB to a resistor-divider
between the output and GND. Select feedback resistor R2 in the 5kΩ to 500kΩ range.
R1 is then given by:
R1 = R2 [(VOUT / VFB) - 1]
where VFB = 1.25V. Add a small ceramic capacitor around 47pF to 100pF in parallel
with R1 to compensate for stray capacitance at the FB pin and output capacitor
equivalent series resistance (ESR).
5.2
Output Capacitor Selection
Choose input- and output-filter capacitors to service inductor currents with acceptable
voltage ripple. The input-filter capacitor also reduces peak currents and noise at the
voltage source. In addition, connect a low-ESR bulk capacitor (>10µF suggested) to the
input. Select this bulk capacitor to meet the input ripple requirements and voltage rating,
rather than capacitor size. Use the following equation to calculate the maximum RMS
input current:
IRMS = IOUT[VOUT (VIN - VOUT)]1/2 · VIN
When selecting an output capacitor, consider the output- ripple voltage and approximate
it as the product of the ripple current and the ESR of the output capacitor.
VRIPPLE = [VOUT (VIN - VOUT)] / [2 · fOSC(L) (VIN)] · ESRCOUT
where ESRCOUT is the equivalent-series resistance of the output capacitor. Higher
values of COUT provide improved output ripple and transient response. Lower oscillator
frequencies require a larger-value output capacitor. Verify capacitor selection with light
loads during PFM operation, since output ripple is higher under these conditions. Low-
ESR capacitors are recommended. Capacitor ESR is a major contributor to output ripple
(usually more than 60%). Ordinary aluminum-electrolytic capacitors have high ESR and
should be avoided. Low-ESR aluminum-electrolytic capacitors are acceptable and
relatively inexpensive. Low-ESR tantalum capacitors are better and provide a compact
Target Datasheet Rev. 1.7
20
2001-09-17