A6902D13TR データシートの表示（PDF） - STMicroelectronics

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A6902D13TR

Up to 1 A step down switching regulator with adjustable current limit for automotive applications

STMicroelectronics 

A6902D

Error amplifiers

5.5

5.5.1

5.5.2

5.6

generally known as “masking time” or “blanking time” (270 ns typ.). The current threshold of the over-current

comparator is given in Table 4. Electrical characteristics.

Both of the protections implement a constant current strategy as the device limits the output current.

The Constant Current Loop, called CCL (see Section 7.2.2 Constant current loop), is adjustable and precise but

slower than the embedded Over Current Protection called “OCP” (see Section 6.2 OCP protection), which is very

fast but with fixed and spreaded current threshold over the population.

The OCP protects the device and external load in case of fast and unexpected line / load transient like a short

circuit event.

Error amplifiers

The current outputs of the two internal error amplifiers are ORed and compared to the oscillator saw tooth to

perform PWM control (voltage mode structure). A compensation network valid for both voltage and current loop is

shared in the COMP pin to achieve adequate phase margin.

Voltage loop

The voltage error amplifier is the core of the loop regulation. It is a transconductance operational amplifier whose

non inverting input is connected to the internal voltage reference (1.235 V) and the inverting input (FB) is

connected to the external divider or directly to the output voltage. The uncompensated error amplifier has the

following characteristics:

Table 5. Uncompensated voltage error amplifier characteristics

Description

Transconductance

Low frequency gain

Minimum sink/source current

Output voltage swing

Input bias current

Value

2300 µS

70 dB

1500 µA / 300 µA

0.4 V / 3.65 V

2.5 µA

Constant current loop

The current error amplifier is a transconductance operational amplifier with a typical offset equal to 100 mV which

directly senses the voltage across the sensing resistor. The uncompensated current error amplifier has the

following characteristics.

Table 6. Uncompensated voltage error amplifier characteristics

Description

Transconductance

Low frequency gain

Minimum sink/source current

Output voltage swing

Value

1900 µS

60 dB

1500 µA / 300 µA

0.4 V / 3.65 V

PWM comparator and power stage

This block compares the oscillator saw tooth and the error amplifier output signals to generate the PWM signal for

the driving stage.

The power stage is a highly critical block, as it functions to guarantee a correct turn ON and turn OFF of the

PDMOS. The turn ON of the power element, or more accurately, the rise time of the current at turn ON, is a very

critical parameter. At a first approach, it appears that the faster the rise time, the lower the turn ON losses.

However, there is a limit introduced by the recovery time of the recirculation diode.

DS5503 - Rev 6

page 8/33

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