CA3272A 데이터 시트보기 (PDF) - Intersil

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CA3272A

Quad-Gated Inverting Power Drivers with Fault Mode Diagnostic Flag Output

Intersil 

CA3272A, CA3292A

The output will continue to turn on and off for as long as the

shorted condition exists or until shutdown by the input logic.

The resulting frequency and duty cycle of the output current

flow is determined by the ambient temperature, the thermal

resistance of the package in the application and the total

power dissipation in the package. Since each output is inde-

pendently protected, the frequency and duty cycle of the cur-

rent flow into multiple shorted outputs will not be related in

time. Long lead lengths in the load circuit may lead to oscilla-

tory behavior if more than two output loads are shorted.

FAULT SENSE

VBATT

4V

THRESHOLD, VTHD

ZENER

CLAMP

TLIM

ZA

QA

RLOAD

2

OUT A

IO(SINK)

ILIM

0.02Ω

FIGURE 3. OUTPUT OPEN LOAD DETECTION WHERE IO(SINK)

IS AN ACTIVE CURRENT SINK PULLDOWN FOR

OPEN-LOAD FAULT DETECTION. THE CURRENT

ICEX IS IO(SINK) PLUS LEAKAGE CURRENTS OF

THE OUTPUT DRIVER

Since a diagnostic flag indicates when an output is shorted,

this information can be used as input to a microprocessor or

dedicated logic circuit to provide a fast switch-off when a

short occurs and, by sequence action, can be used to deter-

mine which output is shorted. A fault condition in any output

load will cause the FAULT output to switch to a logic “low”.

Since a fault condition may be detected during switching,

use of an appropriate size capacitor to filter the FAULT out-

put is recommended. The recommended FAULT output cir-

cuit is shown in Figure 2. This will prevent the FAULT output

voltage from reaching a logic level “0” within the maximum

switching time.

The FAULT detection circuitry compares the state of the

input and the state of the output for each A, B, C and D

channel. The output is considered to be in a high state if the

voltage exceeds the typical FAULT threshold reference volt-

age, VTHD of 4V. If the output voltage is less than VTHD, the

output is considered to be in a low state. For example, if the

input is high and the output is less than VTHD, a normal

“ON” condition exists and the FAULT output is high. If the

input is high and the output is greater than VTHD, a shorted

load condition is indicated and the FAULT output is low.

When the input is low and the output is greater than VTHD, a

normal “OFF” condition is indicated and the FAULT output is

high. If the input is low and the output is less than VTHD, an

open load condition exists and the FAULT output is low. The

Output Driver Fault Sense state is determined by high and

low comparator threshold limits which are defined in the

Fault Parameters section of the Electrical Specifications.

The FAULT output diagram of Figure 2 shows the circuit

component interface for sensing a diagnostic fault condition.

As noted, the time constant of TX = RXCX should be greater

than the ON-OFF output switching times to avoid false fault

readings during switching. For applications requiring fast

period repetition rates, the maximum time constant should

be significantly less than the period of switching. The short-

est practical time constant is preferred to limit the duration of

a fault condition.

To match a standard CMOS or TTL interface, the switched

current at the FAULT pin must be converted to VIH and VIL

voltage levels using the RX external pullup resistor. The min-

imum specified IOL limit at the FAULT output defines the Low

(Fault) state which is used to test for a VOL maximum limit of

0.4V. This makes the calculation for the VIL input level rela-

tively simple. Where VF is the FAULT output voltage, VCC is

the power supply voltage, RX is the pullup resistor to VCC

from the FAULT pin and IOL is the fault condition sink cur-

rent, IO(SINK), the low state equation is:

VF = VCC - RXIOL ≤ VIL

(EQ. 1)

As an example: Since TTL is the worst case for a low state,

VIL = 0.8V. Using VCC = 5V, maximum VF = VOL = 0.4V and

minimum IOL = 1mA for the CA3272A and CA3292A. At the

worst case limit, the minimum value of RX is:

RX = (VCC - VIL)/IOL = (5 - 0.4)V/0.001mA = 4.6kΩ

The preferred value for RX would be greater than the values

calculated.

For the logic VIH High (normal state),

VF = VCC - RXIOH ≥ VIH

(EQ. 2)

Where the IOH current is the specified leakage current,

IF(LK) at the FAULT pin, it remains to check the calculated

value for RX as a leakage current times the chosen pullup

resistance. To determine that the minimum VOH from the

FAULT pin is greater than VIH to an external logic match, VF

is calculated using Equation 2. For example, using the mini-

mum RX resistor value calculated for the CA3272A,

VF = [5 - (4.6kΩ x 20µA)] = 4.9V

which is more than suitable for CMOS or TTL Input switching

levels; suggesting that a larger value of RX (such as 10kΩ)

could be used for a better noise margin in the Low fault state.

To detect an open load, each output has an internal low-level

current sink, shown in Figure 3, which acts as a pull-down

under open load fault conditions and is always active. The mag-

nitude of this current plus any leakage associated with the out-

put transistor will always be less than 100µA. (The data sheet

specification for ICEX includes this internal low-level sink cur-

rent). The output load resistance must be chosen such that the

voltage at the output will not be less than VTHD when the ICEX

sink current flows through it under worse case conditions with

minimum supply voltage. For example, assume a 6.5V mini-

mum driver output supply voltage, a FAULT threshold reference

voltage of VTHD = 5.5V and an output current sink of

ICEX = 100µA. Calculate the maximum load resistance that will

not result in a FAULT output low state when the output is OFF.

6

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