UM1557 datasheet(67/111 Pages) STMICROELECTRONICS

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UM1557

Paralleling of HSDs

Doc ID 023521 Rev 2

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5.2.2

Hybrid HSDs supplied from different supply lines

Paralleling CS pins of hybrid HSDs is possible, however some precautions in the schematic

should be applied if the HSDs are supplied from different supply lines. Direct connection of

CS pins (as shown in Figure 47) is not safe.

Figure 47.

Direct connection of CS pins (not recommended)

Direct connection of CS pins is not safe in the following cases:

●

Loss of Vbat1 or Vbat2

●

Loss of GND connection

Loss of either Vbat1 or Vbat2 leads to an incorrect current sense signal. If Vbat2 is lost, the

U2 logic part is supplied by the U1 current sense signal through the internal Vcc-CS clamp

structure. Consequently the voltage on the CS bus drops, resulting in an inaccurate VSENSE

reading.

If the GND connection of one device is lost, positive as well as negative ISO pulses on the

associated supply line are no longer clamped (considering no other devices connected on

this supply line). If the transient voltage is high enough to activate the involved clamp

structures, there may be an unlimited current flow between both supply lines through the CS

pins. This current can lead to malfunction or even failure of one or both of the HSDs.

In order to ensure a valid current sense signal and to protect devices in all previously

described cases, we may add a diode in series to each CS pin (as previously described in

the case of monolithic devices – see Section 5.1.1: Monolithic HSDs supplied from different

supply lines).

GND

Vbat1

GND

100nF

Vbat2

GND

100nF

GND

Rsense

GND

ADC_IN

Logic

OUT

FR_Stby

GND

Vcc

Reverse

Bat. Prot.

Pwr Clamp

Logic

OUT

FR_Stby

GND

Vcc

Reverse

Bat. Prot.

Pwr Clamp

Isense

Vsense influenced

Loss of Vbat2

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