L9805E datasheet(94/125 Pages) STMICROELECTRONICS

94 / 125 page

On-chip peripherals

L9805E

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Figure 48.

Power bridge schematic

EN bit in PBCSR is the main enable signal, active high. If EN = 0, all the bridge transistors

are switched off (UL, UR, DL and DR are reset) and the outputs OUTL and OUTR are in

high impedance state.

Being '0' the status of EN after reset, the bridge is in safe condition (OUTL=OUTR=Z).

Therefore the safe condition is guaranteed in undervoltage condition (LVD reset) and in

case of main clock (Safeguard reset) or software (Watchdog reset) failures.

Each power DMOS has its own over current detector circuit generating SC_xx signals (see

Figure 48). SC_xx signals are ORed together to generate SC flag in PBCSR register.

SC flag is then set by hardware as soon as one of the two outputs (or both) are short to

battery, ground or if the two outputs are short together (load short). This read only bit is

reset only by clearing the EN bit. The rising edge of SC causes an interrupt request if the

PIE bit is set in PBCSR register.

When the current monitored in any of the four DMOS of the bridge exceeds limit threshold

ISC, the SC bit is set and the corresponding DMOS is switched off after tSCPI time. This

function is dominant over any write from data bus by software (i. e. as long as SC is set, the

bridge cannot be switched on).

To switch as the bridge again the EN bit must be cleared by software. This resets the SC bit.

Setting again EN, the bridge is switched on. If the overcurrent condition is still present, SC is

set again (and an interrupt is generated when enabled).

An internal thermal protection circuit monitors continuously the temperature of the device

and drives the OVT bit in PBCSR register and, in turn, the OVT signal in Figure 48.

The OVT flag is set as soon as the temperature of the chip exceeds Thw and all the

transistors of the bridge are switched off. This rising edge causes an interrupt request if the

PIE bit is set. This read only bit is reset only by clearing the EN bit. This function is dominant

over any write from data bus by software (i. e. as long as OVT is set the bridge cannot be

switched on).

To switch the bridge on again the EN bit must be cleared by software. This resets the OVT

bit. Setting again EN, the bridge is switched on. If the overtemperature condition is still

present, OVT is set again (and an interrupt is generated when enabled).

VBR

PGND

OUTR

VBL

PGND

OUTL

SC_UL

OVT

SC_DL

OVT

SC_DR

OVT

SC_UR

OVT

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