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L9821

HIGH SIDE DRIVER

STMicroelectronics 

L9821

FUNCTIONAL DESCRIPTION

The L9821is a high sidedrive monolithic switch, dri-

ven by TTL, CMOS input logic, able to supply resi-

stive or inductive loads up to 6A DC allowing a

current peak of 25A with a RDS(ON) = 0.1. The elec-

tronicswitch, in additionto its main function,protects

itself, the power network and the load against load

dump (up to 60V) and overload and it detects short

circuit, open load and overtemperature conditions.

All these functions (logic control and power actua-

tion)are possible on a singlechip thanks to the new

mixed ST MultipowerBCD technologythat allows to

integrate isolated DMOS power transistors in com-

binationwith Bipolarand CMOS signal structureson

the same chip.

The high side drive connection (series switch bet-

ween the load and the positive powersource)is par-

ticularlysuited in automotiveenvironmentwherethe

electrochemicalcorrosion withstandinghas primary

importance. For this connection the best solution is

a Power MOS N-channel which requires for driving

only a capacitive charge pump completely integra-

ted on the switch chip.

The L9821 is based on a power DMOS series ele-

ment, a driving circuit with a charge pump, an input

logic interface and on some protectionand fault de-

tection circuits.

The power DMOS transistor has a RDS(ON) = 0.1Ω

(typ. value @ TJ = 25°C, VGS = 10V). The low value

of RDS(ON) is important both to increase the power

transferred to the load and to minimize the power

dissipated in the device.

The charge pump is a capacitivevoltage tripler star-

ting from power supply (car battery), driven by a

500kHz oscillator.

The input interface is based on a circuitry solution

able to guarantee the stability over temperature of

the TTL logic levels and very low quiescent current

in OFF condition.

When the supply reaches the maximum operating

voltage (32V) the device is turned OFF, protecting

itself and the load ; moreoverlocal zener clamps are

provided in some critical points to avoid that VGS of

any MOS transistor could reach dangerous values

even during 60V load dump transient.

The inrush current limiting is a significant feature of

the L9821. This functionallows to protect the power

supplynetwork and may extend the life of the loads.

For example, in the case of the lamps, the tungsten

wire resistance value in cold condition is about one

tenthof the nominalsteadystateand then the inrush

current during the turn on is statistically one of the

main causes of lamps failures. If the high current

condition persists (e.g. load short circuit) and the

junctiontemperaturerises above150°C,the thermal

protectioncircuit turns off the device preventingany

damage. The current limiting and the thermal shut-

down are sufficient to protectthe device againstany

overload becausethe power DMOS has not the se-

cond breakdown.

When the L9821 is driven and one of the protec-

tions (overtemperature, overvoltage, overload) is

present, a fault detection open drain output turns

on. This output is active also when Iload is lower

than 0.6A detectingthe open load (disconnected or

burned out). The diagnostic output detects fault

conditions with 25ms delay in order to avoid spu-

rious diagnosys (i.e. : turn on overcurrent, overvol-

tage spikes etc.). In OFF conditions the fault

detection circuits are not active to allow a minimum

quiescent current.

The device can drive unipolar DC motors and sole-

noids as well becauseit can recirculate an inductive

current when the output voltage goes lower than

Vclamp value (typically - 6.5V in respect to ground).

The possibility to have a start up current is useful

also for DC motors allowing the maximum starting

torque.

TYPICAL APPLICATION OF THE L9821

The L9821 integrated high side driver can be used

to replace an electromechanical relay.

Inductiveload (i.e. solenoids,motors) can be driven

by the L9821. No externalcomponents are required

for the coil current recirculation, because the device

provides this function internally.

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