A3942KLGTR-T Datasheet, PDF (17/20 Pages) Allegro MicroSystems – A3942KLGTR-T for Automotive Applications

A3942
Quad High-Side Gate Driver
for Automotive Applications
That is, the load voltage is within ∆V = I_D × R_D volts
of V_BB.
Using V_DS = V_BB – V_L and rearranging, we find that
V_DS < I_D × R_D ± V_{OS .}
Therefore,
R_D = ⁽V_DS(trip) ±V_OS) / I_D ,
which is also the case for STG faults, described below.
Note that an STB condition generally latches the OL
flag as well.
Under normal conditions R_L << RGS and I_OL flows
through the load, given
I_OL × (R_D + R_L ) < V_BB –I_D × R_D ± V_OS .
Because I_OL × (R_D + R_L ) ≈ 0 when the external
MOSFET is off, no fault is registered.
Short-to-Ground Fault Level The effect of the STG
comparator is to compare the external MOSFET V_DS
(V_L) to the set trip voltage V_BB –I_D × R_D .
The comparator is active only when the gate is com-
manded on. Also, the sourced current I_OL is deacti-
vated.
If V_DS is too large, an STG fault is registered when
V_L < V_BB –I_D × R_D ± V_OS ,
or, because the external MOSFET V_DS = V_BB – V_L,
V_DS > I_D × R_D ± V_OS .
Therefore, the STG trip level in the on state is the
same as the STB level in the off state:
R_D = ⁽V_DS(trip) ±V_OS) / I_D .
Converse to the preceding, in normal operation
V_L > V_BB –I_D × R_D ± V_OS ,
or
Power Limits
Power dissipation, P_D, is limited by thermal con-
straints. The maximum junction temperature, T_J(max),
and the thermal resistance, R_θJA , are given in this
datasheet. The maximum allowed power is then found
for a given ambient, T_A , from this equation:
T_J = P_D × R_θJA + T_A , or
P_D = (T_J – T_A ) / R_θJA .
The three main contributions to power dissipation are:
• quiescent supply, P_BB(Q)
• driver outputs, P_DRV , and
• logic level supply, P_DD .
These three terms appear in the following equation:
P_D = P_BB(Q) + P_DRV + P_DD .
The quiescent supply current leads to a baseline power
loss:
P_BB(Q) = V_BB × I_BB(Q) .
In general, the losses in a driver can be quantified as
follows. Given that the driver current leaves Gx to
charge a gate, and assuming that the external circuit
is approximately lossless, then the same charge is
sunk back into Gx. Therefore, all driver current can be
treated as going to heat the chip.
Total current into V_BB includes the quiescent current,
I_BB(Q) , plus additional current, ∆I_BB, to energize the
gates. The latter is three times the average gate cur-
rent:
∆I_BB = 3 × I_Gx(av) .
The average load current is calculated using the gate
charge, Q_G , from the external MOSFET datasheet and
the switching frequency:
V_DS < I_D × R_D ± V_OS .
I_Gx(av) = f_sw × Q_G .
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