tle6389 Infineon Technologies Corporation, tle6389 Datasheet - Page 18

• Manufacturer Part Number: tle6389
• Description: Step - Down Dc/dc Controller
• Manufacturer: Infineon Technologies Corporation

Target Datasheet Rev. 1.7
4.1
4.1.1
The TLE6389 uses a slope-compensated, current-mode PWM controller capable of
achieving 100% duty cycle. The device uses an oscillator-triggered, minimum on-time,
current-mode control scheme. The minimum on-time is approximately 200ns unless in
dropout. The maximum on-time is greater than 1/fOSC, allowing operation to 100% duty
cycle. Current-mode feedback provides cycle-by-cycle current limiting for superior load-
and line-response and protection of the external MOSFET and rectifier diode. At each
falling edge of the internal oscillator, the oscillator cell sends a PWM ON signal to the
control and drive logic, turning on the external P-channel MOSFET. This allows current
to ramp up through the inductor to the load, and stores energy in a magnetic field. The
switch remains on until either the current-limit comparator is tripped or the PWM
comparator signals that the output is in regulation. When the switch turns off during the
second half of each cycle, the inductor’s magnetic field collapses, releasing the stored
energy and forcing current through the rectifier diode to the output-filter capacitor and
load. The output-filter capacitor stores charge when the inductor current is high and
releases it when the inductor current is low, thus smoothing the voltage across the load.
During normal operation, the TLE6389 regulates output voltage by switching at a
constant frequency and then modulating the power transferred to the load each cycle
using the PWM comparator. A multi-input comparator sums three weighted differential
signals: the output voltage with respect to the reference, the main switch current sense,
and the slope-compensation ramp. It modulates output power by adjusting the inductor-
peak current during the first half of each cycle, based on the output-error voltage.
4.1.2
The TLE6389 operates with a duty cycle up to 100%. This feature extends allows to
operate with the lowest possible drop at low battery voltage as it occurs at cold ambient
temperatures. The MOSFET is turned on continuously when the supply voltage
approaches the output voltage. This services the load when conventional switching
regulators with less than 100% duty cycle would fail. Dropout voltage is defined as the
difference between the input and output voltages when the input is low enough for the
output to drop out of regulation. Dropout depends on the MOSFET drain-to-source on-
resistance, current-sense resistor, and inductor series resistance, and is proportional to
the load current:
Dropout Voltage = I
Operating Modes
PWM Control Scheme
100% Duty-Cycle Operation and Dropout
OUT
x (R
DS(ON)
+ R
SENSE
18
+ R
INDUCTOR
)
TLE 6389
2001-09-17