A6902D Просмотр технического описания (PDF) - STMicroelectronics

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A6902D

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

STMicroelectronics 

6

Additional features and protection

A6902D

Additional features and protection

6.1

Feedback disconnection

If the feedback is disconnected, the duty cycle increases towards the maximum allowed value, bringing the output

voltage close to the input supply. This condition could destroy the load.

To avoid this hazardous condition, the device is turned OFF if the feedback pin is left floating.

6.2

OCP protection

The OCP features two types of current limit protections: pulse-by-pulse and frequency foldback.

The schematic of the current limitation circuitry for the pulse-by-pulse protection is shown in figure below.

The output power PDMOS transistor is split into two parallel PDMOS transistors. The smallest one includes a

resistor in series, RSENSE. The current is sensed through RSENSE and if it reaches the threshold, the mirror

becomes unbalanced and the PDMOS is switched off until the next falling edge of the internal clock pulse. Due to

this reduction of the ON time, the output voltage decreases. The frequency shifter function is enabled by the

overcurrent event and it skips up to three switching pulses depending on the feedback voltage. In strong

overcurrent conditions the device can reduce the switching frequency down to 1/3 of the nominal 250 kHz to keep

the switch current limited.

Figure 6. Current limit circuitry

VCC

A1

DRIVER

RSENSE

A2

OUT

A1/A2=95

I

PWM

RTH

IL

NOT

I

In overcurrent protection mode, when the peak current reaches the current limit, the device reduces the TON

down to its minimum value (approximately 250 ns) and the switching frequency to approximately one third of its

nominal value even when synchronized to an external signal. In these conditions, the duty cycle is strongly

reduced and, in most applications, this is enough to limit the current to ILIM. In any event, in case of heavy

shortcircuit at the output (VOUT = 0 V) and depending on the application conditions (Vcc value and parasitic

effect of external components) the current peak could reach values higher than ILIM. This can be understood

considering the inductor current ripple during the ON and OFF phases:

• ON phase

(1)

∆ IL, TON =

VIN − VOUT −

DCRL + RDSON

L

∙ IL ∙ TON

• OFF phase

(2)

∆ IL, TOFF

=

VD

+ VOUT − DCRL

L

∙ IL

∙

TOFF

DS5503 - Rev 6

page 10/33

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