TLE7368

Detailed Internal Circuits Description

Data Sheet

22

Rev. 1.1, 2007-11-08

5

Detailed Internal Circuits Description

In the following the main circuit blocks of the TLE7368, namely the Buck converter, the linear regulators, the

trackers, the charge pump, the enable and reset circuits and the watchdog are described in more detail.

5.1

Buck Regulator

The TLE7368’s DC to DC converter features all the functions necessary to implement a high efficient, low emission

Buck regulator with minimum external components. The step down regulator in the TLE7368 follows a concept

similar to the one of its predecessor, TLE6368, which allows operation over a battery voltage range from as low

as 4.5 V up to a maximum of 45 V at peak currents of 2.5 A at minimum. Figure 3 shows the block diagram of the

converter with its major components, i.e. the internal DMOS power stages, the high side driver including its supply

scheme, the power stage slope control circuit for reduced EME, the current mode control scheme and various

protection circuits for safe converter operation.

5.1.1

Buck Regulator Control Scheme

The step down converter’s control method is based upon the current mode control scheme. Current mode control

provides an inherent line feed forward, cycle by cycle current limiting and ease of loop compensation. No external

compensation components are needed to stabilize the loop, i.e. the operation of the Buck converter. The slope

compensation circuit in addition to the current sense amplifier and the error amplifier prevents instabilities/sub

harmonic oscillations at duty cycles higher than 0.5. The cycle by cycle current limiting feature supports also a soft

start feature during power up. Additional implemented current blanking prevents faulty DMOS turn off signals

during switching operation.

5.1.2

High Side Driver Supply and 100% Duty Cycle Operation

The supply concept of the Buck converter’s power stage driver follows the Bootstrapping principle. A small external

capacitor, placed between pins SW and BST, is used to provide the necessary charge at the gate of the power

stage. The capacitor is refreshed at each switching cycle while the power stage is turned off resulting in the ability

to power the gate at the next turn on of the power stage.

In cases where the input/battery voltage approaches the nominal Buck converter output voltage, the duty cycle of

the converter increases. At the point where the power stage is statically turned on (100% duty cycle) a refresh of

the Bootstrap capacitor as described above is not possible. In this case the charge pump helps to accomplish the

gate over drive in order to keep the power stage turned on with low

R

before switching to 100% duty cycle, the device operates in pulse skipping mode. In this mode the device appears

to be operating at much lower frequencies with very small duty cycles. In real, the device is doing a few 100% duty

cycle periods followed by a period with a duty cycle smaller than 1.