A2550KLPTR View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
'A2550KLPTR' PDF : 15 Pages View PDF
A2550
Relay Driver with 5 V Regulator
for Automotive Applications
PLS = (RDS(on) × I2LS1) + (RDS(on) × I2LS2)
(9)
+ (RDS(on) × I2LS3) .
Because ILS1 = ILS2 = ILS3 = 110 mA , and
given that RDS(on) = 5 Ω, then
PLS = 3 (5 Ω) * (110 mA) 2 = 182 mW .
Given also:
PD = PBIAS + PREG + PLS
(10)
= 42 mW+ 182 mW+ 182 mW = 406 mW .
TJ can be calculated by substitution into equation 6:
TJ = 0.406 W × 55°C/W +125°C = 147°C .
Reverse Battery
The low-side driver outputs can withstand reverse battery
when the load (RLOADx) is connected to limit current. Power
dissipation (PD = PLS(rvrs)) is limited by thermal constraints,
according to the following formula:
PLS(rvrs) = VF1× IF1+VF2× IF2+VF3× IF3 ,
(11)
where:
= − IFx
VBB(rvrs) VFx
RLOADx
.
(12)
The output voltage is clamped to protect the driver. The
active clamp works as follows. The voltage at the driver out-
put is pushed high by the inductive current. Once the clamp
voltage, VCL, is reached, a Zener diode conducts current to
the internal FET gate driver block. Therefore, the FET turns
partially on, in order to limit any further increase in voltage
at the output pin. The output is then held at this clamp volt-
age until the current decays to zero, as shown in figure 4.
Energy loss in the chip, E, may be calculated as follows.
Load coil resistance, RCOIL , is usually a significant value,
but a worst case scenario takes RCOIL = 0 for simplicity. With
active clamping at VCL, the output current (with initial value
IOUT0) is driven low and the upper limit on energy loss in the
driver is calculated as:
Emax
=
1
2
IOUT0VCL Δt .
(14)
From figure 4:
Δt = LCOILIOUT0 ,
(15)
VCL – Vdc
and
Emax
=
1
2
LCOIL IO2UT0
(1–Vdc
/
VCL
)–1
.
(16)
Active Clamp on Outputs
The driver section includes an active clamp that prevents an
overvoltage when an inductive load is switched off. Zener
diodes are connected at the output pins. This removes the
need for external freewheeling diodes across inductive loads.
The coil current, ICOIL , is quenched by allowing the output
pin voltage, VOUTx , to exceed the battery voltage at the load,
Vdc. This applies a negative voltage drop across the load.
Therefore the current gradient is driven negative, as shown in
the following formula:
dICOIL
dt
=
Vdc
− VOUT − ICOIL×RCOIL
L COIL
<0
.
(13)
IOUT
IOUT0 =
Vdc
RCOIL
m
=
–(VCL – Vdc)
LCOIL
Δt
Figure 4. Output Voltage Clamping
Allegro MicroSystems, Inc.
13
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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