A3944 Datasheet

A3944
Automotive, Low-Side FET Pre-Driver
Description
Features and Benefits
• 6 channels
• Drives logic-level N-channel MOSFETs
• 40 mA gate drive current
• Short and open detection
• High voltage (50 V) drain feedback inputs
• Programmable fault timers and thresholds per channel
• UVLO and thermal warning circuitry
• Serial or parallel gate drive control
• Highly configurable, through SPI compatible interface
• Compact TSSOP package
Applications:
• Automotive ECU
• Automotive high-side actuators
Package: 28-pin TSSOP with exposed
thermal pad (suffix LP)
The A3944 is a programmable 6 channel low-side MOSFET
pre-driver suitable for use in automotive applications. Each
channel is controllable by a combination of parallel and serial
inputs and provides sufficient gate drive current to allow
PWM control up to 10 kHz, depending on the MOSFET gate
charge.
Each channel provides independent fault diagnostics for short
to ground and open load when in the off-state, and short to
battery when in the on-state. A short to battery can disable
the output until reset or for a programmable retry time. Each
channel provides independently programmable fault thresholds
and blanking times.
In addition to channel state control, channel fault masking, fault
thresholds and fault timers are programmed through the SPI
compatible serial interface. The serial interface also provides
read back of the fault status for each channel.
Digital inputs and outputs are compatible with 3.3 V and 5 V
supplies.
The A3944 is supplied in a 28 lead TSSOP package (suffix
LP) with an exposed thermal pad. The package is lead (Pb)
free with 100% matte-tin leadframe plating.
Not to scale
Typical Application Diagram
Parallel
A3944
Controller
SPI
A3944-DS, Rev. 1
A3944
Automotive, Low Side FET Pre-Driver
Selection Guide
Part Number
Packing*
A3944KLPTR-T
4000 pieces per reel
*Contact Allegro™ for additional packing options
Absolute Maximum Ratings with respect to ground at TA = 25°C
Characteristic
Symbol
Notes
Rating
Unit
Analog Supply Voltage
VBB
–0.3 to 40
V
Logic Supply Voltage
VDD
–0.3 to 6.5
V
Gate Drive Supply Voltage
VDR
–0.3 to 6.5
V
Terminal VREG
VREG
–0.3 to 20
V
Terminals GATx
–0.3 to 6.5
V
Terminals DRNx
–0.3 to 50
V
Terminals INx
–0.3 to 6.5
V
Terminals SI, SCK, CSN
–0.3 to 6.5
V
Terminal SO
–0.3 to 6.5
V
Terminal RESETN
–0.3 to 6.5
V
Drain Feedback Clamp Energy*
EDRNC
Single pulse less than 2 ms
10
μJ
Drain Feedback Clamp Current*
IDRNC
Single pulse not exceeding EDRN or PDRN
100
mA
PDRNC
Average power over any 2 ms period
100
mW
150
°C
175
°C
Drain Feedback Clamp
Power*
Junction Temperature
TJ(max)
Transient Junction Temperature*
TtJ
Storage Temperature Range
Tstg
Operating Temperature Range
TA
*Guaranteed
Overtemperature event not exceeding 10 s,
lifetime duration not exceeding 10 hours
Range K
–55 to 150
°C
–40 to 150
°C
by design characterization.
Thermal Characteristics may require derating at maximum conditions, see application information
Characteristic
Symbol
Package Thermal Resistance,
Junction to Ambient
RθJA
Package Thermal Resistance,
Junction to Pad
RθJP
Value
Unit
4-layer PCB based on JEDEC standard
Test Conditions*
28
ºC/W
2-layer PCB with 3.8 in.2 of copper area each side
32
ºC/W
2
ºC/W
*Additional thermal information available on the Allegro website
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
A3944
Automotive, Low Side FET Pre-Driver
Pin-out Diagram
28 VDD
VDR 1
GAT0 2
27 RESETN
DRN0 3
26 SO
GAT1 4
25 SCK
DRN1 5
24 SI
GAT2 6
DRN2 7
PAD
23 CSN
22 IN0
GAT3 8
21 IN1
DRN3 9
20 IN2
GAT4 10
19 IN3
DRN4 11
18 IN4
GAT5 12
17 IN5
DRN5 13
16 GND
VREG 14
15 VBB
Terminal List Table
Name
Number
Function
Name
Number
Function
CSN
23
Serial interface chip select
IN1
21
Gate drive 1 control input
DRN0
3
Gate drive 0 drain sense input
IN2
20
Gate drive 2 control input
DRN1
5
Gate drive 1 drain sense input
IN3
19
Gate drive 3 control input
DRN2
7
Gate drive 2 drain sense input
IN4
18
Gate drive 4 control input
DRN3
9
Gate drive 3 drain sense input
IN5
17
Gate drive 5 control input
DRN4
11
Gate drive 4 drain sense input
PAD
–
Exposed thermal pad, connect to ground
DRN5
13
Gate drive 5 drain sense input
RESETN
27
Chip reset input
GAT0
2
Gate drive 0 output
SCK
25
Serial clock
GAT1
4
Gate drive 1 output
SI
24
Serial data input
GAT2
6
Gate drive 2 output
SO
26
Serial data output
GAT3
8
Gate drive 3 output
VBB
15
Analog supply (Battery)
GAT4
10
Gate drive 4 output
VDD
28
Logic Supply
GAT5
12
Gate drive 5 output
VDR
1
Gate drive supply
GND
16
Power ground
VREG
14
Voltage regulator
IN0
22
Gate drive 0 control input
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
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A3944
Automotive, Low Side FET Pre-Driver
Functional Block Diagram
IN5 IN4 IN3 IN2 IN1 IN0
VBB
VDR
Regulator
GND
VDD
VDD
Config
Register
Gate Drive Channel
Control
Registers
Config
Register
CSN
SO
SI
SCK
Gate Drive Channel
SPI
GND
Config
Register
Fault
Register
Gate Drive Channel
Config
Register
Fault Logic
RESETN
Gate Drive Channel
POR
UVLO
Config
Register
Gate Drive Channel
Retry Timer
Config
Register
Clock
Gate Drive Channel
VREG
DRN0
GAT0
DRN1
GAT1
DRN2
GAT2
DRN3
GAT3
DRN4
GAT4
DRN5
GAT5
GND
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115 Northeast Cutoff
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A3944
Automotive, Low Side FET Pre-Driver
VBAT
VOCL
Off
Blanking
Fault
Filter
Fault
Decode
Fault
Filter
+
-
Off-State
Fault
Detect
Load
IDPU
65 μA
VOL
+
-
VSTG
DRNx
RDX
50 V
Fault
Filter
On-State
Fault
Detect
On
Blanking
+
-
VSTB
VOCL
Disable
Retry
on STB
tRE
IDPD
65 μA
VDR
GATx
RESETN
POR
RGx
INx
Gx
VSTB VSTG VOL
Channel
Configuration
Register
Threshold
Generator
VREG
Figure 1. Gate Drive Channel functional block diagram (shows 1 channel of 6)
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115 Northeast Cutoff
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A3944
Automotive, Low Side FET Pre-Driver
ELECTRICAL CHARACTERISTICS Valid at TJ = –40°C to 150°C, VDD = 3.3 V, VDR = 5 V, VBB = 6 to 40 V; unless
otherwise specified
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
3.0
–
5.5
V
Supply and Reference
Logic Supply Voltage
VDD
Analog Supply Voltage
VBB
6
–
40
V
Gate Drive Supply Voltage
VDR
3.0
–
6.0
V
VDD Quiescent Current
IDDQ
–
–
3
mA
–
–
4
mA
–
–
10
μA
VBB Quiescent Current
IBBQ
VDR Quiescent current
IDRQ
Regulator Voltage
VREG
Regulator Dropout
RESETN Pulse Width
VDD = 0, VBB ≤ 30 V
–
–
4
mA
17.5
–
18.5
V
VDO
0
–
0.6
V
tRST
1
–
–
μs
VBB > 19.5 V
RESETN Glitch Filter
tRGF
–
–
200
ns
Oscillator Frequency
fOSC
1.4
2
2.6
MHz
VIH
70
–
–
%VDD
Digital Inputs and Outputs
Input High Voltage
Input Low Voltage
VIL
–
–
30
%VDD
Input Hysteresis
VIhys
300
500
–
mV
Input Pull-Up Resistor
RPU
CSN to VDD
–
50
–
kΩ
Input Pull-Down Resistor
RPD
INx, SI, SCK to GND
SO Output High Voltage*
VOH
SO, IOH = –2 mA
SO Output Low Voltage
VOL
SO Output Leakage*
IL
–
50
–
kΩ
VDD - 0.4
VDD - 0.2
–
V
SO, IOL = 2 mA
–
0.2
0.4
V
CSN = VDD
–1
–
1
μA
TJ = 25°C, IGHx = –20 mA
25
50
70
Ω
TJ = 150°C, IGHx = –20 mA
50
75
125
Ω
TJ = 25°C, IGLx = 20 mA
25
50
70
Ω
Gate Output Drive
Pull-Up On-Resistance*
Pull-Down On-Resistance
RDS(on)UP
RDS(on)DN
TJ = 150°C, IGLx = 20 mA
50
75
125
Ω
Output Sink Current
IGL
GATx off, VGATx = VDR
20
–
–
mA
Output Source Current*
IGH
GATx on, VGATx = 0 V
–
–
–40
mA
Output Rise Time
tr
CLOAD = 400 pF, 20% to 80% VDR
–
180
–
ns
Output Fall Time
tf
CLOAD = 400 pF, 80% to 20% VDR
–
180
–
ns
Minimum On-Time
ton
At INx input
–
–
1
μs
Minimum Off-Time
toff
At INx input
–
–
1
μs
tP(on)
INx to GATx
–
200
–
ns
INx to GATx
–
200
–
ns
RESETN to GATx
–
0.5
1
μs
Turn-On Propagation Delay
Turn-Off Propagation Delay
tP(off)
Continued on the next page…
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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A3944
Automotive, Low Side FET Pre-Driver
ELECTRICAL CHARACTERISTICS (continued) Valid at TJ = –40°C to 150°C, VDD = 3.3 V, VDR = 5 V, VBB = 6 to 40 V; unless
otherwise specified
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
IDRNx = 10 μA
45
–
–
V
Fault Detection (On-State)
Drain Clamp Voltage
VDCL
Drain Clamp Leakage
IDC
Short to Battery Threshold
Retry Timer
Fault Filter Time
Fault Blank Timer
VSTB
tRE
IDRNx = 10 mA
–
54
–
V
VDRNx < 32 V
–
–
1
μA
GATx driven high, SB[2:0] = 111
30
31
32
%VREG
GATx driven high, SB[2:0] = 110
17
18
19
%VREG
GATx driven high, SB[2:0] = 101
15
16
17
%VREG
GATx driven high, SB[2:0] = 100
13
14
15
%VREG
GATx driven high, SB[2:0] = 011
11
12
13
%VREG
GATx driven high, SB[2:0] = 010
9
10
11
%VREG
GATx driven high, SB[2:0] = 001
7
8
9
%VREG
GATx driven high, SB[2:0] = 000
5
6
7
%VREG
RT0 = 1
40
55
72
ms
RT0 = 0
7
10
13
ms
1.25
2
2.75
μs
GATx driven high, TON[1:0] = 11
40
56
72
μs
GATx driven high, TON[1:0] = 10
20
28
36
μs
GATx driven high, TON[1:0] = 01
10
14
18
μs
GATx driven high, TON[1:0] = 00
4
5
7
μs
tFF(on)
tBL(on)
Fault Detection (Off-State)
DRNx Pull-Up Diagnostic Current*
IDPU
GATx low, VDRNx < (VOCL – 200 mV)
–80
–65
–50
μA
DRNx Pull-Down Diagnostic Current
IDPD
GATx low, NPD = 0, VDRNx >(VOCL + 200 mV)
50
65
80
μA
Short to Ground Threshold
VSTG
GATx driven low, SG = 1
65
66
67
%VREG
GATx driven low, SG = 0
44
45
46
%VREG
GATx driven low
75
76
77
%VREG
70
71
72
%VREG
Open Load Threshold
VOL
Open Load Clamp Voltage
VOCL
Fault Filter Time
tFF(off)
Fault Blank Timer
tBL(off)
1.25
2
2.75
μs
GATx driven low, TOF[1:0] = 11
3000
4000
5000
μs
GATx driven low, TOF[1:0] = 10
200
280
360
μs
GATx driven low, TOF[1:0] = 01
100
140
180
μs
GATx driven low, TOF[1:0] = 00
60
80
100
μs
Continued on the next page…
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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A3944
Automotive, Low Side FET Pre-Driver
ELECTRICAL CHARACTERISTICS (continued) Valid at TJ = –40°C to 150°C, VDD = 3.3 V, VDR = 5 V, VBB = 6 to 40 V; unless
otherwise specified
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
50
–
–
ns
Serial Interface Timing
Clock High Time
tSCKH
A in figure 2
Clock Low Time
tSCKL
B in figure 2
50
–
–
ns
CSN Set-up to SCK Low
tCSS
C in figure 2
30
–
–
ns
CSN Hold after SCK High
tCSHD
D in figure 2
30
–
–
ns
CSN High Time
tCSH
E in figure 2
300
–
–
ns
Data Out Enable Time
tSOE
F in figure 2
–
–
40
ns
Data Out Disable Time
tSOD
G in figure 2
–
–
30
ns
Data Out Valid Time from
Clock Falling
tSOV
H in figure 2
–
–
40
ns
Data Out Hold Time from
Clock Falling
tSOH
I in figure 2
5
–
–
ns
Data In Set-up Time to Clock Rising
tSIS
J in figure 2
15
–
–
ns
Data In Hold Time from Clock Rising
tSIH
K in figure 2
10
–
–
ns
CSN High to Output Change
tPCS
–
200
–
ns
2.6
2.75
2.9
V
50
100
150
mV
4.5
4.8
5.1
V
100
200
300
mV
Chip Diagnostics Protection
VDD Undervoltage Lockout
VDDUV
VDD Undervoltage Lockout
Hysteresis
VDDUVhys
VREG Undervoltage Lockout
VREGUV
VREG Undervoltage Lockout
Hysteresis
VREGUVhys
Overtemperature Warning Threshold
Overtemperature Hysteresis
Decreasing VDD
Decreasing VREG
TJW
Temperature increasing
145
160
175
ºC
TJWhys
Recovery = TJW – TJWhys
–
15
–
ºC
*For input and output current specifications, negative current is defined as coming out of (sourcing) the specified device terminal.
...
...
CSN
C
A
B
SCK
J
SI
X
K
D15
F
SO
D 14
X
I
Z
D14
D15
X
...
...
D
X
D0
E
X
G
D0
Z
H
X = Don’t care, Z = High impedance (tri-state)
Figure 2. Serial Interface Timing Diagram. Letter keys refer to the Serial Interface Timing section of the Electrical
Characteristics table.
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115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
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A3944
Automotive, Low Side FET Pre-Driver
Functional Description
The A3944 provides a programmable interface between an ECU
and 6 low-side MOSFET switches in automotive applications.
Each channel provides all the features necessary to drive and
monitor the external FET and load.
than the minimum reset pulse width: forces outputs low, resets
the configuration, sets the LR bit, and resets all other channel
faults.
SI: Active high digital input with pull-down resistor. Data on SI is
clocked into the serial register on the rising edge of SCK.
The gate of the external FET is driven by a 50 Ω (typ) push-pull
driver capable of sourcing and sinking at least 40 mA under all
conditions. This is sufficient to allow most typically used FETs to
be switched with a PWM input at up to 10 kHz. The state of each
channel is determined by a combination of parallel and serial
inputs.
SCK: Digital clock input with pull-down resistor. See SI and SO
for action.
When the FET is active its drain is monitored for a short to
battery. When the FET is inactive, internal current sources are
activated and the drain voltage is monitored to check for shorts to
ground or open loads.
CSN: Active low digital input with pull-up resistor. When CSN
is low SO becomes active and data is accepted on SI. Data is
latched in the serial register when CSN goes high. When CSN is
high SO is high impedance and SI and SCK are ignored.
The serial, SPI compatible interface provides access to control
and configuration registers. Each channel has a dedicated fault
configuration register that allows independent fault thresholds,
fault timing, and fault configuration for each channel.
INx: Active high digital inputs with pull-down resistors. When
The output state of each channel is determined by the logic control input for the channel and a dedicated bit in the single output
control register. Channels can therefore be controlled by parallel
input, by serial input, or by a combination of the two. All channels can be switched at the same time with a single serial write.
GATx: Gate drive outputs. Drive between GND and VDR. Con-
A single fault mask register can be used to ignore the fault detect
output for any channel combination.
The serial interface also provides read back of the fault status for
each channel.
Digital inputs and outputs are compatible with 3.3 V and 5 V
supplies.
Terminal Functions
VDD: Positive supply for digital input, output, and logic.
VBB: Positive supply for voltage regulator. Can be connected to
battery voltage through reverse polarity protection.
VREG: Regulated voltage for analog and reference functions.
VDR: Positive supply for gate drive outputs.
GND: Ground return. Connect to common return point for all
external MOSFET source connections.
RESETN: Active low digital input. When held low for longer
SO: Push-pull digital output. Data from the fault register is output
on SO, changing on the falling edge of SCK.
INx is high GATx is allowed to go high, depending the contents
of the serial control register and any active faults. When INx is
low GATx is held off.
nected through a resistor or directly to the gate of the external
MOSFETs.
DRNx: Analog, high-voltage inputs. Drain monitor connection
used to determine the status of the drive to the load.
Gate Drive Channels
Each gate drive channel has independent control logic, gate drive
output, fault detection circuitry, fault threshold generators, fault
timers, and fault configuration register. The fault configuration
register and reference generation provides two short to ground
thresholds and eight short to battery thresholds, plus four turn-on
blank times and four turn-off blank times independently selectable per channel.
The gate drive channel block diagram (figure 1) shows the functional circuit for one gate drive channel, which is duplicated in
each gate drive channel. A retry timer, common to all channels,
allows automatic retry for short to battery faults.
Control and Enable
A gate drive output, GATx, is turned-on when: RESETN is high,
no short to battery fault is present, and either the direct digital
input, INx, or the relevant bit in the serial control register, Gx, is
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A3944
Automotive, Low Side FET Pre-Driver
high, in other words the logical OR of the INx input and the Gx
bit for each channel x. If the GATx output is to be controlled by
the serial interface, then the corresponding INx logic input should
be held low. Internal pull-down resistors from each INx terminal
to GND ensure that any unconnected input will be pulled low.
Conversely, if the GATx output is to be controlled by the INx
logic input, then the corresponding Gx bit in the control register
should be set to 0, which is its default power-on and reset state.
Gate Drive Output
Each gate drive output is designed to provide symmetrical charge
current from the VDR supply terminal and discharge current to
the GND return terminal. The maximum source and sink impedance provides peak charge and discharge currents of at least
50 mA when connected directly to the gate of the external FET.
This current can be limited, in order to limit the FET turn-on
switching speed, by using a resistor between the GATx output
and the gate of the FET. Although the GATx drive is designed to
be symmetrical, the actual drive performance will be affected by
the FET parameters and the resistance between the GATx output
and the gate of the FET.
The VDR supply is used only to supply the GATx output. The
voltage at VDR can therefore be varied to provide voltage limited
drive to the FET gate. Undervoltage detection is not provided for
this supply.
Reset Function
If RESETN is held low for more than the minimum reset pulse
width, then all registers are reset to their power-on state, and all
GATx outputs are held low. Any latched channel faults and corresponding bits in the fault register are reset, the logic reset (LR)
bit is set, and the UV and OT bits reflect the status of the undervoltage and overtemperature detectors.
The RESETN input uses a glitch filter to reduce the susceptibility
to transients and noise on the RESETN input. This glitch filter
is guaranteed to ignore any pulses shorter than the minimum
RESETN glitch filter time, tRGF .
Channel Fault Diagnostics
All channel faults are determined by monitoring the voltage at the
drain of the external FET through the DRNx terminal. Each channel has independent bias current generators, programmable fault
comparators, fault decode logic, and programmable fault timers.
The serial interface provides a dedicated fault configuration register for each channel to select these features and thresholds per
channel. A single fault mask register provides a fault mask bit for
each channel. Fault detection is disabled when RESETN is low or
when the fault mask bit is set. Fault reporting through the serial
interface is fully described in the Serial Interface section below.
A short to battery (short across the load to the load supply) can be
detected when the channel is active, GATx is high, and the FET
is on (on-state). A short to battery fault always attempts to protect
the FET by pulling GATx low.
A short to ground or open load can be detected when GATx is
low and the FET is off (off-state). A short to ground fault or
open load fault does not interfere with the operation of the GATx
output.
Each channel fault detected is latched as a fault state, and remains
latched until the diagnostic circuits can determine that the fault
has been removed for that channel. This determination can only
occur at the end of a fault blank time. For short to battery this is
at the end of the on-state fault blank time following a transition
from off to on. For a short to ground or open load this is at the
end of the off-state fault blank time following a transition from
on to off.
When a fault is detected, a dedicated bit in one of the two fault
registers is set for each fault on each channel. This requires 3 bits
per channel over 6 channels or 18 fault bits in total. The fault bits
in the fault registers remain latched until the first serial transfer
after the associated fault state has been reset. All latched fault
states and all latched channel fault bits can also be cleared either
by a power-on reset or by taking the RESETN terminal low.
Practical limits for load resistance, and voltage conditions to
provide effective determination of the load status, are discussed
in the Applications Information section below.
Note that each DRNx terminal has an internal Zener clamp which
limits the voltage at the terminal to VDCL . If the voltage at the
drain of the FET is likely to be higher than VDCL , even during a transient, then a current limit resistor, RDx , must be added
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10
A3944
Automotive, Low Side FET Pre-Driver
between the drain connection to the FET and the DRNx terminal.
This resistor should be selected such that the energy dissipated by
the clamp diode is less than the absolute maximum drain clamp
energy, EDRN . This is necessary to avoid excessive heat generation in the silicon; otherwise permanent damage to the chip is
likely. Selecting a value for RDx is described in the Applications
Information section, below.
On-State Diagnostics: Short to Battery
When a channel is in the on-state the voltage at the drain monitor
terminal, VDRNx , is compared to a threshold level derived from
the voltage at the REF terminal. A short to battery fault is present
if VDRNx is higher than VSTB (see figure 3). Note that an open
FET also would be detected as a short to battery.
The threshold voltage, VSTB , is selected per channel as a percentage of the voltage at the REF terminal. The voltage selection is
determined by the SB0, SB1, and SB2 bits (bits1, 2, and 3 of the
fault configuration register for the channel).
When a FET is switched-on there is a finite time before the drain
voltage reaches a steady state. To avoid false fault detection at
switch-on, the output from the short to battery comparator is
ignored during the on-state fault blank time, tBL(on) , after the
GATx output is commanded to drive high. One of four possible
on-state fault blank times is selected, per channel, through the
VDRNx
On-State
Diagnostics
Off-State
Diagnostics
Normal
Operation
Short to Battery
Detected
VDRNx
VBB
VOL
Open Load
Detected
VOCL
VSTG
VSTB
Normal
Operation
Figure 3. Diagnostic Threshold Voltages
Short to Ground
Detected
TON0 and TON1 bits (bits 4 and 5 of the fault configuration
register for the channel).
To avoid false fault detection during supply transients, when the
FET is active an additional fault filter will mask faults that are
present for less than the on-state fault filter time, tFF(on) . This
fault filter is only active after the fault blank time.
The result is that directly after switch-on, a short to battery fault
will not be detected until tBL(on) after the GATx output is commanded to drive high. If a short occurs after tBL(on) from switchon, then it must be present for at least tFF(on) before it is detected.
When a short to battery is detected the GATx output is automatically commanded to drive low and to switch-off the FET. Two
alternative modes are then available depending on the status of
the RT0 and RT1 bits (bits 10 and 11 of the fault configuration
register for the channel).
If RT1 is 0, the FET will be held off until the fault is reset by
pulsing RESETN low for longer than tRST or by toggling the
channel off then on, through the serial interface. In this mode,
toggling the input terminal for the channel has no effect until
after a reset.
If RT1 is 1, the channel will be held off until one of the two
common retry timers completes a time-out. The timer selection is
made by the state of the RT0 bit. The channel control bits, both
serial and logic input, are ignored during this time. At the end of
the retry time-out the channel will be switched-on again if the
state of the control logic commands the channel to be on. In the
retry mode the fault can be reset also by pulsing RESETN low for
longer than tRST or by toggling the channel off then on, through
the serial interface. This resets the channel fault and re-enables
the channel from the control logic.
Note that, if the common retry timer has already been activated
by another channel, then the first retry time-out for the second
channel may be shorter that the full time. Subsequent retry
sequences will run for the full time-out period minus the short
detection time.
Off-State Diagnostics: Open Load and Short to Ground
Two current generators and two comparators per channel provide
off-state diagnostic capability. If the voltage at the DRNx terminal, VDRNx , is greater than the open load clamp voltage, VOCL ,
then one of the current generators sinks current to VOCL through
the DRNx terminal. If VDRNx is less than VOCL , then one of the
current generators sources current from VOCL through the DRNx
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A3944
Automotive, Low Side FET Pre-Driver
terminal. The voltage output capability of the current sources is
limited such that they cannot source current when the output voltage is greater than VOCL or sink current when the output voltage
is less than VOCL. The equivalent circuit is shown in figure 4a.
The typical sink and source currents are shown graphically in
figure 4b.
When a channel is in the off-state the current generators source or
sink current through the DRNx terminal in an attempt to pull the
voltage at the terminal to the open load clamp voltage, VOCL . The
resulting voltage at the DRNx terminal, VDRNx , is measured to
test for a short to ground or an open load.
Normal Operation
If a load is present, the load supply is active, and there are no
shorts to ground, then the load will pull VDRNx above VOCL .
IDPD
IDRNx
VOCL
VDRNx
GND
Figure 4a. Diagnostic Current Source Circuit
100
80
NPD = 0
60
IDPD
IDRNx (μA)
40
20
NPD = 1
0
–40
IDPU
–100
0
2
4
6
VDRNx (V)
Figure 4b. Diagnostic Currents
8
VOCL
–80
10
Short to Ground
If a short to ground is present, then VDRNx will be pulled low by
the short circuit. VDRNx will be less than the short detect threshold, VSTG , and a short to ground fault is reported (see figure 3).
The short detect threshold voltage, VSTG , is selected, per channel,
as a percentage of the voltage at the VREG terminal. The voltage
selection is determined by the SG bit (bit 0 of the fault configuration register for the channel).
The time taken for VDRNx to reach the correct value for an open
fault condition depends on the current sourced from the DRNx
terminal and on the capacitance connected to the drain of the
FET. To avoid false fault detection at switch-off, the outputs
from the short to ground and open load comparators are ignored
during the off-state fault blank time, tBL(off) , after the GATx output drives low. One of four possible off-state fault blank times is
selected, per channel, through the TOF0
and TOF1 bits (bits 6 and 7 of the fault configuration register
for the channel).
To avoid false fault detection during supply transients, an additional fault filter masks faults that are present for less than the
off-state fault filter time, tFF(off) . This fault filter is only active
after the fault blank time.
–20
–60
The open load threshold, VOL , and the open load clamp voltage,
VOCL , are a both a fixed percentage of the voltage at the VREG
terminal and are common to all channels.
Open Load
If there is no short to ground or to supply, and the load is not connected, then the current sources will pull VDRNx towards the open
load clamp voltage, VOCL . VDRNx will then be greater than VSTG
but less than VOL and an open load is reported (see figure 3).
DRNx
IDPU
VDRNx will then be greater than the open load threshold, VOL ,
and no fault is detected (see figure 3). If the NPD bit is zero then
the current sink will pull IDPD from the supply through the load,
through the DRNx terminal, to VOCL
12
The result is that directly after switch-off, a short to ground or
open load fault will not be detected until tBL(off) after the GATx
output is commanded to drive low. If a fault occurs after tBL(off)
from switch-off then it must be present for at least tFF(off) before it
is detected.
In some applications, for example when driving high efficiency
LEDs, the load may be sensitive to the pull down current used to
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A3944
Automotive, Low Side FET Pre-Driver
ensure open load detection. In these cases this pull-down current
can be disabled by setting the NPD bit (bit 8 in the fault configuration register for the channel). If the NPD bit is set then it is
possible that VDRNx will reach the correct value for an open fault
condition when the load is connected, resulting in a false open
load detection.
If this is likely, there are two options:
• Set the fault mask bit for the channel. This will mask all faults
on that channel and may not be a suitable option.
• Set the open load fault mask bit, OLM (bit 6 in the fault mask
register). This will disable open load detection on any channel
where NPD is set to 1.
Chip Diagnostics
The chip temperature and the supply voltage levels at VDD and
VREG are monitored to ensure correct and safe operation of the
circuit.
VDD is monitored to ensure that power-up and power-down does
not cause incorrect operation. All outputs will be switched to high
impedance, the VREG regulator will be disabled and all faults
reset when the voltage at VDD, VDD , falls below the undervoltage level, VDDUV . The outputs will be reactivated when VDD
rises above the undervoltage turn-on level plus the hysteresis
voltage, defined as VDDUV + VDDUVhys . When VDD rises above
this threshold, all registers will be reset to their power-on state,
and all GATx outputs will be low. In the fault register any latched
channel faults will be reset, the logic reset (LR) and undervoltage
(UV) bits will be set, and the OT bit will reflect the status of the
overtemperature monitor.
VREG is monitored to ensure correct operation of the fault
detection and control circuits. All channel faults will be reset
when the voltage at VREG, VREG , falls below the undervoltage
level, VREGUV . They will be held reset until VREG rises above the
undervoltage lockout level plus the hysteresis voltage, VREGUV
+ VREGUVhys . The outputs will remain active irrespective of the
value of VREG .
The chip temperature is monitored by the thermal warning circuit. An overtemperature fault will be indicated but no action will
be taken when the chip temperature exceeds the overtemperature
warning level TJW . It is incumbent upon the user to take any necessary action to limit dissipation to reduce the temperature.
Serial Interface
The inputs CSN, SCK, and SI provide a three wire synchronous
serial interface, compatible with SPI, that can be used to control
all features of the A3944. The output, SO, can be used to provide
a fourth interface connection for detailed diagnostic feedback.
The serial interface timing requirements are specified in the Electrical Characteristics table, and illustrated in the Serial Interface
Timing diagram, figure 2. Data is received on the SI terminal and
clocked through a shift register on the rising edge of the clock
signal input on the SCK terminal. CSN is normally held high, and
is only brought low to initiate a serial transfer. No data is clocked
through the shift register when CSN is high, allowing multiple
slave units to use common SI, SCK, and SO connections. Each
slave then requires an independent CSN connection.
When 16 data bits have been clocked into the shift register, CSN
must be taken high to latch the data into the selected register.
When this occurs, the internal control circuits act on the new data
and the fault register is reset.
If there is either: more than 16 rising edges on SCK, or at least
one but fewer than 16 rising edges on SCK and CSN goes high,
then the write will be cancelled without writing data to the registers or resetting the diagnostic registers. The FF bit will be set to
indicate a data transfer error.
Configuration and Control Registers
The serial data word is 16 bits, input MSB first. The first four bits
are defined as the register address. This provides sixteen writeable registers:
Address 1: Gate Select Register
The six least significant bits of this register are the control bits
for each of the six channels. G0 corresponds to channel 0, G1 to
channel 1, and so forth. If RESETN is high and no faults are present on the channel, then when the Gx bit for a channel is set to 1
the GATx output will be high.
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A3944
Automotive, Low Side FET Pre-Driver
Address 5: Fault Mask Register
The six least significant bits of this register are the fault mask bits
for each of the six channels. K0 corresponds to channel 0, K1 to
channel 1, and so forth. When the K bit for a channel is set to 1
all faults on that channel are ignored and no faults are reported
for that channel. Bit 6 is an open load fault mask bit, OLM, that
disables the open load detection on any channel where NPD is
set to 1.
Addresses 8 to 13: Channel Fault Configuration Registers
These six registers, one per channel, determine the fault threshold
levels, fault blank times, and fault features for each channel. The
MSB is always set to 1. The next three bits, bits 12,13, and 14,
are the channel address bits.
The remaining register addresses are unused. Writing to these
addresses will have no effect on the operation but will still report
the fault register on SO.
Fault Register
In addition to the writable registers there are two fault registers,
Fault0 and Fault1. The register being output is identified by bit
11, which contains a zero for Fault0 and a one for Fault1. Each
time any register is written through the serial interface, one of
the fault registers can be read, MSB first, on the serial output
terminal, SO (see the Serial Interface Timing diagram, figure 2).
Fault0 is output: on the first write after a power-on-reset, after a
RESETN low input, or after a serial fault poll (described in the
next paragraph). Fault1 is then read on the next write. The two
registers then alternate on each successful serial write sequence.
The first, most significant, bit in both fault registers is the fault
register flag, FF (bit 15). This bit is set to one, if any faults have
been detected since the last fault reset. The state of FF appears
on SDO as soon as CSN goes low, allowing the fault status to be
determined without a change in the level of SCK. A serial transfer may be terminated when CSN goes low then high, without
generating a serial read fault, by ensuring that SCK remains high
while CSN is low. This allows the main controller to poll the
A3944 through the serial interface to determine if a fault has been
detected. When this occurs the fault register pointer is reset to the
Fault0 register, so the next full write sequence outputs the Fault0
register. The fault status can also be read, without disturbing any
settings, by writing to one of the unused register addresses. In this
case the fault registers will continue to alternate between Fault0
and Fault1.
The next three most significant bits, after FF, in each fault register are the system diagnostic bits: UV (bit 14), LR (bit 13), and
OT (bit 12). These provide an indication of undervoltage, logic
reset, and overtemperature faults.
Bit 11 (FR) indicates which of the two fault registers is being
output on SO. This bit is a zero for the Fault0 register and a one
for the Fault1 register.
The least significant 9 bits in each fault register provide three
bits per channel, one bit for each of the three possible fault states:
short to battery, short to ground, and open load. Fault0 contains the fault data for channels 0, 1, and 2. Fault1 contains the
fault data for channels 3, 4, and 5. The bits naming convention
indicates the channel and fault allocation. The format is “ccff,”
where “cc” is C0, C1, C2, C3, C4, or C5 (indicating the channel)
and “ff” is SG, SB, or OL (indicating the faults: short-to-ground,
short-to-battery and open-load respectively).
The contents of the fault register that is being read cannot change
when CSN is low and a serial read is in progress. Any faults
detected during a serial read do affect the read in progress but the
fault will be latched in the fault register when CSN goes high at
the end of a serial read. The fault bits can only be cleared after:
either the diagnostic circuits have confirmed that the fault that
has been reported is no longer present, or there is a low level on
the RESETN input.
In the case of undervoltage or overtemperature faults, which are
not latched, the fault bits will be reset at the end of a serial read if
the fault is not detected at that time.
For channel faults, which are latched, the fault bits will be reset at
the end of a serial read if the diagnostic circuits have previously
determined that the fault that has been reported is no longer present. This determination can only occur at the end of a fault blank
time and therefore requires: either an on-to-off transition (for
short to ground and open load faults), or an off-to-on transition
(for short to battery faults) on the faulty channel before the start
of a serial read. Any changes to the fault state when a read is in
progress are ignored until the end of the serial read. If a fault is
cleared when a serial read is in progress, then the fault bits will
be cleared when CSN goes high.
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A3944
Automotive, Low Side FET Pre-Driver
Figures 5 through 10 are channel fault timing diagrams, which
show fault conditions applied to a channel and the results of the
fault latches and the state of the fault register after serial reads
and channel state changes. Each diagram shows the state of
the channel control signal, INx, and the state of the gate drive
output, GATx, for the channel, as well as an approximation of
the relative voltage, DRNx, at the drain of the external MOSFET.
Beneath this is the latched fault state and the contents of the fault
register bits for the channel. The sequence of serial reads are
shown at the bottom of each figure as the state of the CSN input
and the resulting data bits read for the channel.
Output
Disabled
tRE
INX
GATx
DRNx
Fault
Short
Applied
tBL(on)
None
Fault register bit
CxSB
Short
Removed
tBL(on)
Short to Battery
0
VSTB
tBL(on)
None
1
0
CSN
Serial Read
Serial Read Data
Cxff=0
CxSB=1
CxSB=1
CxSB=1
CxSB=1
Cxff=0
Figure 5. Fault Sequence: Short to battery during off-state (RT1=1)
INX
GATx
Short
Applied
DRNx
Short
Removed
tBL(off)
Fault
None
Fault register bit
CxSG
VSTG
tBL(off)
tBL(off)
Short to Ground
0
None
1
0
CSN
Serial Read
Serial Read Data
Cxff=0
Cxff=0
CxSG=1
CxSG=1
CxSG=1
Cxff=0
Figure 6. Fault Sequence: Short to ground during on-state
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Automotive, Low Side FET Pre-Driver
INX
GATx
tBL(off)
tBL(off)
Fault
Load
Connected
Load
Removed
DRNx
None
Fault register bit
CxOL
Open Load
0
VOL
VOCL
VSTG
tBL(off)
None
1
0
CSN
Serial Read
Cxff=0
Serial Read Data
Cxff=0
CxOL=1
CxOL=1
CxOL=1
Cxff=0
Figure 7. Fault Sequence: Open load during on-state
Output
Disabled
tRE
INX
GATx
tFF(on)
DRNx
Fault
None
Fault register bit
CxSB
Short
Removed
Short
Applied
tBL(on)
Short to Battery
0
VSTB
tBL(on)
None
1
0
CSN
Serial Read
Serial Read Data
Cxff=0
CxSB=1
CxSB=1
CxSB=1
CxSB=1
Cxff=0
Figure 8. Fault Sequence : Short to battery during on-state (RT1=1)
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Automotive, Low Side FET Pre-Driver
Short
Applied
INX
Short
Removed
GATx
DRNx
tFF(off)
Fault
None
Fault register bit
CxSG
Short to Ground
0
VSTG
tBL(off)
None
1
0
CSN
Serial Read
Cxff=0
Serial Read Data
Cxff=0
CxSG=1
CxSG=1
CxSG=1
Cxff=0
Figure 9. Fault Sequence: Short to ground during off-state
Output
Disabled
INX
GATx
tFF(on)
tBL(on)
Short
Applied
DRNx
VOCL
VSTB
Load
recovers
Load goes
open circuit
tBL(off)
Faults
None
None
Fault register bits
CxSB
CxOL
tBL(off)
Short to Battery
None
Open Load
0
0
None
1
1
0
1
0
0
CSN
Serial Read
Serial Read Data
Cxff=0
CxSB=1
CxSB=1
CxSB=0
CxSB=0
Cxff=0
Figure 10. Fault Sequence: Short to battery during on-state followed immediately by open load (RT1=0)
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Automotive, Low Side FET Pre-Driver
VDDUV+Hys
VDDUV
VDD
VBB
VREG
VBB/VREG
VREGUV+Hys
VREGUV
Outputs
Z
Enabled
Fault register bits
LR
UV
U
U
0
1
0
0
LR=1
UV=1
LR=0
UV=1
1
1
Z
U
U
Enabled
1
1
Z
U
U
0
1
0
CSN
Serial Read
Serial Read Data
LR=0
UV=0
LR=1
UV=1
LR=0
UV=1
Where Z=high impedance, U=undefined
Figure 11. Power Sequence Timing- VDD before VBB
VDDUV+Hys
VDDUV
VDD
VBB
VREG
VBB/VREG
RESETN
X
Outputs
Z
Fault register bits
LR
UV
CSN
U
U
X
Enabled
1
1
0
0
Off
1
0
Enabled
Z
0
0
U
U
X
X
Serial Read
Serial Read Data
LR=1
UV=1
LR=0
UV=0
LR=1
UV=0
LR=1
UV=0
LR=0
UV=0
Where Z=high impedance, U=undefined, X=don’t care
Figure 12. Power Sequence Timing- VDD after VBB - with RESET
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A3944
Automotive, Low Side FET Pre-Driver
Serial Register Definition*
Gate Select
Fault Mask
Channel Fault Config
Fault0
Fault1
15
14
13
12
0
0
0
1
0
1
1
0
11
10
9
8
7
6
0
0
0
0
0
0
0
0
0
0
0
RT1
RT0
NPD
TOF1
TOF0
0
0
0
0
0
1
ADR2 ADR1 ADR0
0
FF
UV
LR
OT
FR
0
0
1
UV
1
OT
0
0
0
FF
UV
LR
OT
FR
0
0
1
UV
1
OT
1
0
0
C2SG C2SB
0
0
C5SG C5SB
0
0
5
4
3
2
1
0
G5
G4
G3
G2
G1
G0
0
0
0
0
0
0
OLM
K5
K4
K3
K2
K1
K0
0
0
0
0
0
0
0
SB2
SB1
SB0
SG
0
0
0
0
TON1 TON0
0
0
C2OL C1SG C1SB
0
0
0
C5OL C4SG C4SB
0
0
0
C1OL C0SG C0SB
0
0
0
C4OL C3SG C3SB
0
0
0
C0OL
0
C3OL
0
*Power on reset value shown below each input register bit.
Gate Select Register
G[5..0]
Control bits for each of the six channels.
G0 corresponds to channel 0, G1 to channel 1 etc.
If RESETN is high and no faults are present on
the channel then when the Gx bit for a channel is
set to 1 the GATx output will be high
Fault Mask Register
K[5..0]
OLM
Fault mask bits for each of the six channels.
K0 corresponds to channel 0, K1 to channel 1 etc.
When the Kx bit for a channel is set to 1 all faults
on that channel are ignored and no faults are
reported for that channel.
Open load fault mask for all channels where
NPD=1. If the NPD bit is set to 1 on a channel
and OLM is set to 1 then the open load diagnostic
is disabled for that channel.
Fault0/Fault1 Registers
FF
Logic 1 if any faults have been detected since the
last fault reset.
UV
Logic 1 if any VDD or VREG undervoltage faults
have been detected since the last fault reset.
LR
Logic 1 if a logic reset has occurred since the last
register read. A logic reset is caused by a poweron-reset or by taking the RESETN input low.
OT
Logic 1 if an overtemperature fault has been
detected since the last fault reset.
FR
Fault register identifier.
Logic 0 for Fault0 register,
logic 1 for Fault1 register.
CxSG
Logic 1 if channel short to ground detected.
Where x is channel number.
CxSB
Logic 1 if channel short to battery detected.
Where x is channel number.
CxOL
Logic 1 if channel open load detected.
Where x is channel number.
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A3944
Automotive, Low Side FET Pre-Driver
Serial Register Definition*
15
Channel Fault Config
1
14
13
12
ADR2 ADR1 ADR0
11
10
RT1
RT0
0
0
9
0
8
7
6
NPD
TOF1
TOF0
0
0
0
5
4
TON1 TON0
0
0
3
2
1
0
SB2
SB1
SB0
SG
0
0
0
0
*Power on reset value shown below each input register bit.
Channel Fault Config Register
ADDR[2..0]
ADR2
0
0
0
0
1
1
RT[1..0]
RT1
0
1
1
NPD
Channel address
ADR1
0
0
1
1
0
0
ADR0
0
1
0
1
0
1
Address
Channel 0
Channel 1
Channel 2
Channel 3
Channel 4
Channel 5
TON[1..0] Turn-on blank time select.
TON1
0
0
1
1
SB[2..0]
SB2
0
0
0
0
1
1
1
1
Retry select
RT0
X
0
1
Fault
Lockout until reset
Short retry timer. Nominally 10ms
Long retry timer. Nominally 55ms
Disable diagnostic pull-down
NPD
0
1
Action
Enable diagnostic pull-down
Disable diagnostic pull-down
TOF[1..0] Turn-off blank time select.
TOF1
0
0
1
1
TOF0
0
1
0
1
Turn-off blank time (nominal)
80μs
140μs
280μs
4ms
SG
TON0
0
1
0
1
Turn-on blank time (nominal)
5μs
14μs
28μs
56μs
Short to battery threshold select
SB1
0
0
1
1
0
0
1
1
SB0
0
1
0
1
0
1
0
1
Threshold (nominal)
6% VREG
8% VREG
10% VREG
12% VREG
14% VREG
16% VREG
18% VREG
31% VREG
Short to ground threshold select.
SG
0
1
Threshold (nominal)
45% VREG
66% VREG
For tolerances on selected parameters refer to the Electrical
Characteristics Table.
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A3944
Automotive, Low Side FET Pre-Driver
Applications Information
Drain Feedback Clamp Resistor Selection
The drain feedback input, DRNx, for each channel is clamped
internally with a 50 V (nominal) Zener diode. If the voltage
applied to this terminal is likely to exceed 50 V then an external
current limit resistor will be required to limit the current, power,
and energy to less than the Absolute Maximum specifications in
this document.
Note that the internal drain clamp in the A3944 is not intended to
dissipate the energy from any external load. The internal clamp
is provided to protect the internal circuits of the A3944 from any
high voltage that would otherwise cause permanent damage.
If the voltage at DRNx, VDRNx , will never exceed the minimum
drain clamp voltage, VDCL , then no external resistor is required
and DRNx can be connected directly to the drain of the external
MOSFET switch.
These values would typically apply to a remote load which is
primarily resistive. The load inductance will be due to a combination of the wiring and any parasitic inductance in the load. In this
example, the DC on-state current will be 13 V / 26 Ω = 0.5 A.
When the load is switched off, the inductance attempts to keep
the current flowing by increasing the voltage at the end connect
to the FET switch. This voltage increases up to the breakdown
voltage of the FET. At that point, the voltage across the load
amounts to the difference between the FET breakdown voltage
and the supply voltage, and it acts to reduce the current. With the
parameters in this example, the current would decay to zero in
less than 1 μs. This is less than the 1.85 μs limit for maximum
current, so the drain resistor will be based only on the maximum
current. The value of the drain resistor, RDx , in this case is simply
the voltage across the resistor divided by the maximum current:
Three Absolute Maximum specifications apply to the A3944,
none of which may be exceeded:
• The maximum clamp current, IDRNC , applies to very short
pulses, typically less than 1.85 μs. Any current pulse less than
1.85 μs and less than IDRNC , will never exceed the maximum
power or energy limits.
RDx =
VFET – VDCL
IDRNC
where
VFET is the FET breakdown voltage,
VDCL is the A3944 drain clamp voltage, and
• The maximum clamp energy, EDRNC , applies to pulses between
1.85 μs and 2 ms. Above 2 ms the heat produced by the clamp
energy dissipates through the silicon and the package; in that
case, the maximum clamp power applies. Note that for pulse
lengths between about 500 μs and 2 ms the energy starts to dissipate during the pulse, so the maximum current that is possible
will actually be higher than that calculated using the maximum
energy limit.
Substituting into equation 1:
• The maximum clamp power, PDRNC , applies to pulses lasting
longer than 2 ms up to continuous operation.
The energy injected into the A3944 drain clamp is:
Maximum current example:
• Load resistance: 26 Ω
where tPULSE is the duration of the current pulse.
• Load inductance: 130 μH
Substituting into equation 2:
• Load current: 0.5 A
• Load supply voltage: 13 V
• FET clamp voltage: 80 V
(1)
IDRNC is the A3944 drain clamp max current.
RDx =
80 V – 54 V
= 260 Ω
100 mA
EDRNC = VDCL × IDRNC × tPULSE
(2)
EDRNC = 54 V × 100 mA × 0.9 μs = 4.86 μJ (per pulse)
As expected, based on the pulse length, this is less than half the
clamp energy limit, given in the Absolute Maximum table.
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A3944
Automotive, Low Side FET Pre-Driver
The maximum repetition rate of this pulse is derived from the
maximum average clamp power dissipation limit. The minimum
time between pulses, tREP , is:
EDRNC
tREP = P
DRNC
(3)
Substituting into equation 4:
IDRNC =
As given in equation 1, the value of the drain resistor is the voltage across the resistor divided by the maximum current:
where PDRNC is the A3944 drain clamp maximum power.
Substituting into equation 3:
4.86 μJ
tREP =
= 48.6 μs
100 mW
10 μJ
= 18.5 mA
54 V × 10 μs
RDx =
VFET – VDCL
IDRNC
RDx =
80 V – 54 V
= 1.4 kΩ
18.5 mA
resulting in a repetition rate of just over 20 kHz.
As given in equation 3, the maximum repetition rate of this pulse
is derived from the maximum average clamp power dissipation
limit as:
Maximum energy example:
• Load resistance: 18 Ω
• Load inductance: 1 mH
EDRNC
tREP = P
DRNC
• Load current: 0.72 A
• Load supply voltage: 13 V
• FET clamp voltage: 80 V
These values would typically apply to a small inductive load
such as a solenoid or relay. When the load is switched off, the
inductance attempts to keep the current flowing by increasing
the voltage at the end connected to the FET switch. This voltage
increases up to the breakdown voltage of the FET. At that point,
the voltage across the load amounts to the difference between
the FET breakdown voltage and the supply voltage, and it acts to
reduce the current. With the parameters in this example, the current would decay to zero in about 10 μs. This is greater than the
1.85 μs pulse time defining the maximum current but less than
the 2 ms time constant for maximum average clamp power, so
the drain resistor will be selected to limit the energy injected into
the drain clamp in the A3944. The maximum current, the A3944
drain clamp maximum current, IDRNC , will be:
IDRNC =
EDRNC
VDCL × tPULSE
where
EDRNC is the A3944 drain clamp maximum energy,
VDCL is the A3944 drain clamp voltage, and
tPULSE is the duration of the current pulse.
(4)
tREP =
10 μJ
= 100 μs
100 mW
:
resulting in a repetition rate of 10 kHz.
Maximum power example:
• Load resistance: 5 Ω
• Load inductance: 80 mH
• Load current: 2.6 A
• Load supply voltage: 13 V
• FET clamp voltage: 60 V
These values would typically apply to a large inductive load
such as a coil or actuator. When the load is switched off, the
inductance attempts to keep the current flowing by increasing
the voltage at the end connected to the FET switch. This voltage
increases up to the breakdown voltage of the FET. At that point,
the voltage across the load amounts to the difference between
the FET breakdown voltage and the supply voltage, and it acts
to reduce the current. With the parameters in this example, the
current would decay to zero in about 4 ms. This is greater than
the 2 ms time constant for maximum average clamp power, so
the drain resistor will be selected to limit the power dissipated by
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A3944
Automotive, Low Side FET Pre-Driver
the drain clamp in the A3944. The maximum current, the A3944
drain clamp maximum current, IDRNC , will be:
PDRNC
IDRNC =
(5)
VDCL
PDRNC is the A3944 drain clamp maximum power and
VDCL is the A3944 drain clamp voltage.
Substituting into equation 5:
100 mW
54 V
An open load is detected when:
VDRNx < VOL
= 1.8 mA
where VOL is the open load detect voltage, then:
As given in equation 1, the value of the drain resistor is the voltage across the resistor divided by the maximum current:
RDx =
VFET – VDCL
IDRNC
RDx =
60 V – 54 V
= 3.3 kΩ
1.8 mA
where VOCL is the open load clamp voltage.
Ideally an open load would mean an infinite or at least a very
large (>1 MΩ) resistance. In practice this is not necessarily the
case, and the limit of open-load resistance values for correct
detection will be determined by: the threshold voltages, the diagnostic currents, and the load voltage.
where
IDRNC =
Note that this equation is only valid for normal load and open
load conditions when:
VDRNx > VOCL
The maximum repetition rate is irrelevant in this case because the
A3944 will sustain the maximum clamp dissipation indefinitely.
⇒ VOL > VDRNx ,
⇒ VOL > VL – IDPD( RL + RD ) ,
∴ RL > [(VL – VOL) / IDPD ] + RD .
(from 6)
(7)
There are two open-load resistance values to consider. The first is
the minimum resistance at which an open load detection is always
guaranteed. The second is the maximum resistance that a load can
present without ever causing an open load to be detected. Both
cases, described below, assume that the load is connected to the
load supply and that the load supply is higher than the open load
clamp voltage, VOCL.
VL
Practical Open Load Limits
+
VDRNx = VL – IDPD( RL + RD )
-
where
VDRNx is the voltage at the DRNx terminal,
(6)
Load
An open load is detected, when the external FET is off, if the
voltage at the DRNx terminal is less than the open load threshold,
VOL , but greater than the short to ground threshold, VSTG . The
voltage at the DRNx terminal in the off-state is defined as (referring to figure 13):
RL
DRNx
RD
VOL
IDPD
VL is the load supply voltage,
IDPD is the diagnostic pull-down current,
VOCL
RL is the load resistance, and
RD is the DRNx current limit resistor.
Figure 13. Open load detection condition
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A3944
Automotive, Low Side FET Pre-Driver
Minimum guaranteed open load resistance
The minimum value of RL that will always be detected as an open
is given by the maximum value of RL that could be detected as a
load, RLmax. This is defined by:
VLmax – VOLmax
(8)
RLmax =
– RDmin
IDPDmin
For an 18 V supply this gives a minimum guaranteed open load
resistance value of 79 kΩ. This means that under all conditions,
with a load voltage of up to 18 V, a load resistance greater than
79 kΩ will always be detected as an open load. For a 36 V supply
the minimum guaranteed open load resistance value increases to
379 kΩ.
For a 6 V supply this gives a maximum value of 19 kΩ for the
sum of the open load resistance and the DRNx current limit resistor. This means, for example, that under all conditions, with a
DRNx current limit resistor of up to 7 kΩ, a load resistance less
than 12 kΩ will never cause an open load detection.
The two limiting values are shown in figure 14 for load voltages from 6 to 36 V. Note that the load resistance value includes
the DRNx current limit resistor. In this figure, a load resistance
greater than the upper line is guaranteed to be detected as an open
load and a load resistance less than the lower line is guaranteed
not to be detected as an open load.
Practical Short to Ground Limits
Maximum load resistance
The maximum value of RL that will always be detected as a load
is given by the minimum value of RL that could be detected as a
open, RLmin. This is defined by:
RLmax =
VLmax – VOLmax
– RDmin
IDPDmin
(9)
A short to ground is detected, when the external FET is off, if
the voltage at the DRNx terminal is less than the short to ground
threshold, VSTG . Under ideal conditions a short circuit would be
zero resistance and the short would be to ground at zero volts.
However in practical systems the short will have a finite resistance and the power ground voltage may be higher than the reference ground of the detection circuit. The equivalent circuit during
a short to ground is shown in figure 15.
VL
Load
VOCL
IDPU
DRNx
+
-
RL
VSTG
RDx
RSG
VG
Figure 14. Open load detection limits
Figure 15. Short to ground detection condition
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A3944
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The voltage at the DRNx terminal, VDRNx , in the off-state when a
short to ground is present is defined as:
(VL – VG) RSG
VDRNx =
+ VG + IDPURD
RL + RSG
(10)
short detection criterion, RLmax., defined by:
RLmax =
VL – VGmax
VSTGmin – VGmax – IDPUmaxRDmax
– 1 RSGmax (12)
Assuming worst case conditions of a maximum ground voltage
offset of +1 V and a maximum short resistance of 0.5 Ω allows
the minimum load resistance to be calculated for different load
voltages. This will be maximum at either VLmax or VLmin depending on the relative values of RDmax and VL .
where
VL is the load supply voltage,
VG is the ground (offset) voltage,
RSG is the resistance of the short to ground (offset),
For example, at a load voltage of 6 V and RD set to 5 kΩ, a short
will be detected with a load resistance greater than 0.75 Ω when
SG = 0, or at 2.63 Ω when SG = 1. At a load voltage of 18 V the
same conditions give 0.9 Ω for SG = 0 and 0.34 Ω for SG = 1.
RL is the load resistance,
IDPU is the diagnostic pull-up current, and
RD is the DRNx current limit resistor.
Note that this equation is only valid for short to ground conditions when:
VDRNx < VOCL
where VOCL is the open load clamp voltage.
The limiting values for SG = 0 and SG = 1 are shown in figure 16
for load voltage from 6 to 36 V and with a DRNx current limit
resistor (RDmax) of 7 kΩ. In this figure, a load resistance above
the line is guaranteed to allow detection of a 0.5 Ω short to a
+1 V offset ground. An increase in RD raises the line.
A short to ground is detected when:
VD < VSTG
where VSTG is the short to ground detect voltage, then:

(VL – VG) RSG
+ VG + IDPURD < VSTG
RL + RSG
Note that RD must be less than:
(VSTG – VG)
IDPU

<
(VL – VG) RSG
< VSTG – VG – IDPURD
RL + RSG
VL – VG
– 1 RSG
RL >
VSTG – VG – IDPURD
(11)
Minimum load resistance
The minimum value of RL that will always allow a short to be
detected is given by the maximum value of RL that satisfies the
Figure 16. Short to ground detection limits
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A3944
Automotive, Low Side FET Pre-Driver
This will be at a minimum when
Practical Short to Battery Limits
A short to battery is detected, when the external FET is on, if the
voltage at the DRNx terminal is greater than the short to battery
threshold, VSTB . Under ideal conditions a short circuit would be
zero resistance. However in practical systems the short will have
a finite resistance. The equivalent circuit during a short to battery
is shown in figure 17.
The voltage at the DRNx terminal in the on-state when a short to
battery is present is defined as:
VDRNx =
VL RON
RLRSB
+ RON
RL + RSB
(13)
where
(VL – VSTB )
VSTB RL – (VL – VSTB ) RON
is at its minimum. This occurs when VL–VSTB is at its minimum
and VSTB is at its maximum. This is the condition that is present
when VL is just high enough to provide the minimum drop-out
voltage above the maximum value of VREG , (that is, when VL =
VREGmax + VDOmin ) and VSTB is at the maximum tolerance value.
Placing these limits into the expression for RSB gives the expression for the minimum short resistance, defined by:
RSBmax(min) =
VDRNx is the voltage at the DRNx terminal,
(VREGmax+VDOmin+VSTBmax ) RON RL
VSTBmax (RL + RON) – (VREGmax+VDOmin ) RON
(16)
VL is the load supply voltage,
The maximum short resistance at any load voltage is given by:
RON is the FET switch on-resistance,
RL is the load resistance, and
RSB is the resistance of the short across the load.
RSBmax =
A short to ground is detected when:
(VL – VSTBmax ) RON RL
(17)
VSTBmax (RL + RON) – VLRON
VDRNx > VSTB
where VSTB is the short to ground detect voltage, then:a
VL
(14)
RSB
For short to battery diagnostics there are two limiting resistance
values to consider. The first is the maximum value of RSB that
will always cause a short to be detected. The second is the minimum value of RL that will not cause a short to battery detection
under normal operating conditions.
RL
DRNx
+
-
Load

VL RON
> VSTB
RLRSB
+ RON
RL + RSB
VSTB
RON
Maximum short resistance
The maximum value of the short resistance, RSB , that will always
cause a short to be detected is given by the minimum value of
RSB that satisfies the short detection criterion defined by:
RSB <
(VL – VSTB ) RONRL
VSTB (RL + RON ) – VLRON
Figure 17. Short to battery detection condition
(15)
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A3944
Automotive, Low Side FET Pre-Driver
The variation of RSBmax with load voltage is shown as the lower
line (Maximum Short Resistance) in figure 18. This example
shows the maximum possible resistance of a short to battery
that will always be detected as a short with a 2.8 Ω load and a
100 mΩ MOSFET using short to battery threshold level 5.
As RL increases, RSB becomes the dominant resistance and:
VL RON
VD =
RLRSB
+ RON
RL + RSB
VD =
VL RON
RSB + RON
This allows a lower threshold to be used for VSTB , resulting in a
faster short to battery detection and a lower short circuit current.
Minimum load resistance
For normal operation:
VDRNx =
RSB <
VSTB
(20)
RL + RON
and:
(18)
VDRNx < VSTB
Rearranging gives:
The expression for RSB (from equation 15) becomes:
(VL – VSTB )RON
VL RON
RL >
(19)
(VL – VSTB )RON
VSTB
(21)
The minimum value of the load resistance, RL, that will not cause
a short to battery detection under normal operating conditions is
given by:
RLmin =
(VLmax – VSTBmin )RONmax
VSTBmin
(22)
The variation of RLmin with load voltage is shown as the upper
line (Minimum Load Resistance) in figure 18. This example
shows the minimum possible load resistance that will always
allow a short to battery to be detected with a 0.5 Ω short and a
100 mΩ MOSFET using short to battery threshold level 5.
Power Dissipation Estimation
Figure 18. Short to battery detection limits
The A3944 supply currents have very little dependency on the
state of the internal circuits. In addition, the internal operation
is essentially low speed so the power dissipation has almost no
dependency on operating frequency other than dissipation due
to channel switching and diagnostics that are proportional to
PWM frequency. It is therefore possible to estimate the maximum
power dissipated within the A3944 by summing the contribution
from the three quiescent supply currents with the dissipation due
to channel switching and diagnostics associated with turning each
external MOSFET on and off.
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A3944
Automotive, Low Side FET Pre-Driver
Quiescent Dissipation
The quiescent dissipation for each supply is the simply product of
the supply current and the supply voltage:
PDD = VDD × IDDQ
PBB = VBB × IBBQ
PBR = VDR × IDRQ
From the Electrical Characteristics table specification this gives
the total maximum quiescent dissipation of 131 mW when VDD
and VDR are 5 V and VBB is 24 V. At 12 V this drops to 83 mW.
Channel Switching Dissipation
The dissipation produced by switching each channel on or off is
calculated by summing the energy passing through the gate drive
output to and from the gate of the external MOSFET over time.
The energy transferred to the gate is given by:
ESW =
Qg VG
(23)
2
where
Qg is the total MOSFET gate charge and
As an example, the maximum likely switching losses can be
estimated by using a reasonably large MOSFET total charge of
100 nC and a PWM frequency of 10 kHz. With no gate resistor
the dissipation in the A3944 due to switching losses for a single
channel will be approximately 5 mW.
Channel Diagnostic Dissipation
Each channel has three current generators that are used to determine the state of the load during the off-state for the channel.
Under normal load conditions the power dissipated is limited to
the product of the pull-down current source and the difference
between the load supply and the open load clamp voltage. For
example, with a 24 V load supply, this would contribute a maximum of 80 μA × 9.2 V = 0.8 mW. At 12 V this drops to 0.2 mW.
However, the worst case dissipation will occur when the load
is not connected and a capacitor is attached to the diagnostic
feedback terminal for the channel, DRNx. As for the switching
losses, the dissipation can be calculated by summing the energy
transferred to the capacitor over time. In this case the energy
transferred is:
VG is the MOSFET gate voltage when on.
This is the energy transferred through the gate drive each time
a MOSFET is switched on or off. The total power due to this
energy transfer is calculated by multiplying the energy by the
number of switching events per second. The number of switching
events per second is twice the PWM frequency, so the dissipation
due to switching losses becomes:
PSW = Qg VG fPWM
(24)
where fPWM is the PWM frequency for the channel.
If there is no gate resistor then this is the total dissipation that will
occur inside the A3944. If a gate resistor is used then the dissipation will be shared proportionally by the gate resistor and by the
on-resistance of the A3944 gate drive. This gives the equation for
internal dissipation as:
PSW = Qg VG fPWM
RON
RON + RG
where
RON is the on-resistance of the gate drive and
RG is the gate resistor value.
(25)
ED =
CD V 2OCL
2
(26)
where
CD is the value of the DRNx capacitor and
VOCL is the offset clamp voltage.
This is the energy transferred through the current source each
time a MOSFET is switched off. The total power due to this
energy transfer is calculated by multiplying this energy by the
number of switching events per second. The number of switching
events per second is the PWM frequency, so the dissipation due
to switching losses becomes:
PD =
CD V 2OCL fPWM
2
where fPWM is the PWM frequency for the channel.
(27)
As an example, a 10 nF capacitor and a PWM frequency of
10 kHz will produce a dissipation in the A3944 for a single channel of approximately 4.3 mW. This is the worst case dissipation.
It will not be present if a load is attached and will be reduced by
any DRNx current limit resistor.
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A3944
Automotive, Low Side FET Pre-Driver
Total Dissipation Example
The total dissipation is the sum of the quiescent dissipation and
the dissipation due to switching and diagnostic currents in each of
the six channels:
Pmax = PDD + PBB + PDR + 6 (PSW + Pdiag)
(28)
The worst case maximum dissipation occurs at maximum supply
voltage and all loads open circuit. Assuming: a channel PWM frequency of 10 kHz on each channel, a 10 nF capacitor attached to
each DRNx terminal, 10 nC MOSFETs, no DRNx resistors, and
no gate resistors, then the maximum dissipation will be 275 mW.
This is a conservative maximum dissipation showing that the
A3944 can easily be used in high ambient temperatures without
requiring derating.
This worst case dissipation will drop to 227 mW with a 24 V supply and to 139 mW with a 12 V supply.
The maximum typical dissipation, with all loads connected and
the same conditions, will be 219 mW at 36 V, 165 mW at 24 V
,and 114 mW at 12 V (see figure 19).
Figure 19. Power dissipation all loads connected
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A3944
Automotive, Low Side FET Pre-Driver
Package LP, 28-Pin TSSOP
with Exposed Thermal Pad
0.45
9.70±0.10
28
0.65
28
8º
0º
0.20
0.09
1.65
B
3 NOM
4.40±0.10
3.00
6.40±0.20
6.10
0.60 ±0.15
A
1
2
1.00 REF
5.08 NOM
0.25 BSC
Branded Face
28X
SEATING
PLANE
0.10 C
0.30
0.19
0.65 BSC
SEATING PLANE
GAUGE PLANE
C
1 2
5.00
C
PCB Layout Reference View
For Reference Only; not for tooling use (reference MO-153 AET)
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
1.20 MAX
0.15
0.00
A Terminal #1 mark area
B
Exposed thermal pad (bottom surface); dimensions may vary with device
C
Reference land pattern layout (reference IPC7351
SOP65P640X120-29CM);
All pads a minimum of 0.20 mm from all adjacent pads; adjust as
necessary to meet application process requirements and PCB layout
tolerances; when mounting on a multilayer PCB, thermal vias at the
exposed thermal pad land can improve thermal dissipation (reference
EIA/JEDEC Standard JESD51-5)
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A3944
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Revision History
Revision
Revision Date
Rev. 1
May 18, 2012
Description of Revision
Update RDS(on) , IBBQ , and IGL
Copyright ©2011-2013, Allegro MicroSystems, LLC
Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to
permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that
the information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its
use; nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
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