TLE6389_07 Datasheet, PDF (31/38 Pages) Infineon Technologies AG – Step-Down DC/DC Controller

TLE 6389
7.8
SYNC Input and Frequency Control
The TLE 6389’s internal oscillator is set for a fixed PWM switching frequency of 360kHz
or can be synchronized to an external clock at the SYNC pin. When the internal clock is
used SYNC has to be connected to GND. SYNC is a negative-edge triggered input that
allows synchronization to an external frequency ranging between 270kHz and 530kHz.
When SYNC is clocked by an external signal, the converter operates in PWM mode until
the load current drops below the PWM to PFM threshold. Thereafter the converter
continues operation in PFM mode.
7.9
Shutdown Mode
Connecting ENABLE to GND places the TLE6389-2 GV in shutdown mode. In
shutdown, the reference, control circuitry, external switching MOSFET, and the oscillator
are turned off and the output falls to 0V. Connect ENABLE to voltages higher than 4.5V
for normal operation. As this input operates analog the voltage applied at this pin should
have a slope of 0.5V/3µs to avoid undefined states within the device.
7.10
Buck converter circuit
A typical choice of external components for the buck converter circuit is given in figure
2 and 3. For basic operation of the buck converter the input capacitors C_IN1, C_IN2, the
driver supply capacitor C_BDS, the sense resistor R_SENSE, the PMOS device, the catch
diode D1, the inductance L1 and the output capacitor C_OUT are necessary. In addition for
low electromagnetic emission a Pi-filter at the input and/or a small resistor in the path
between GDRV and the gate of the PMOS may be necessary.
7.10.1 Buck inductance (L1) selection in terms of ripple current:
The internal PWM/PFM control loop includes a slope compensation for stable operation
in PWM mode. This slope compensation is optimized for inductance values of 47µH and
Sense resistor values of 47mΩ for the 5V output voltage versions. When choosing an
inductance different from 47µH the Sense resistor has to be changed also:
-R----^S---^E---^N----^S---^E-
L1
=
(
0,5...
1,0
)
×10³
Ω_---
H
Increasing this ratio above 1000 Ω/H may result in sub harmonic oscillations as well-
known for peak current mode regulators without integrated slope compensation.
Datasheet Rev. 2.1
31
2007-08-13