TLE6389 Datasheet, PDF (19/23 Pages) Infineon Technologies AG – Step-Down DC/DC Controller

TLE 6389
4.1.3
PWM/PFM Operation
This control scheme overrides PWM mode and places the TLE6389 in PFM mode at
light loads to improve efficiency and reduce quiescent current to less than 100µA. The
pulse-skipping PFM operation is initiated when the peak inductor current drops below 0.1
x I_limit. During PFM operation, the TLE6389 switches only as needed to service the load,
reducing the switching frequency and associated losses in the external switch, the
rectifier diode, and the external inductor. During PFM mode, a switching cycle initiates
when the PFM comparator senses that the output voltage has dropped too low. The P-
channel MOSFET switch turns on and conducts current to the output-filter capacitor and
load until the inductor current reaches the PFM peak voltage limit. Then the switch turns
off and the magnetic field in the inductor collapses, forcing current through the
synchronous rectifier to the output filter capacitor and load. Then the TLE6389 waits until
the PFM comparator senses a low output voltage again.
Output ripple is higher during PFM operation. A larger output-filter capacitor can be used
to minimize ripple.
4.2
SYNC Input and Frequency Control
The TLE6389’s internal oscillator is set for a fixed-switching frequency of 360kHz or can
be synchronized to an external clock. Do not leave SYNC unconnected. Connecting
SYNC to GND enables PWM/PFM operation to reduce supply current at light loads.
SYNC is a negative-edge triggered input that allows synchronization to an external
frequency ranging between 270kHz and 450kHz. 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.
4.3
Shutdown Mode
Connecting ENABLE to GND places the TLE6389GV, TLE6389G33 and TLE6389G50
in shutdown mode. In shutdown, the reference, control circuitry, external switching
MOSFET, and the oscillator are turn off and the output falls to 0V. Connect ENABLE to
IN for normal operation.
4.4
SI_Enable
Connecting SI_ENABLE to 5V causes SI_GND to have low impedance. Thus the SI
comparator is operational and ca be used to monitor the battery voltage. SO output
signal is valid. Connecting SI_ENABLE to GND causes SI_GND to have high
impedance. Thus the SI comparator is not able to monitor the battery voltage. SO output
signal is invalid.
Target Datasheet Rev. 1.7
19
2001-09-17