SA5778 Datasheet, PDF (10/16 Pages) NXP Semiconductors – Serial triple gauge driver STGD

Philips Semiconductors
Serial triple gauge driver (STGD)
Product specification
SA5778
Sample Calculations for Power Dissipation and
Thermal Management
Worst Case Example
The worst case example will occur when the STGD is operating at
V_BATTMAX (16V, in the highest specified ambient temperature
(105°C), and with the lowest specified coil resistance (171 ohms at
25°C). Typical coil self heating of 15°C is assumed.
Calculation of Coil resistance operating at 105°C ambient.
RCA_{= RCN (1+(0.4%W/°C)*((TSH+Tamb)–25°C))}
= 171 x(1+(0.4%((15+105)–25)))
= 236 Ohms at T_amb=105°C, with 15°C of self heating.
Calculation of STGD power dissipation at 16 volt operation.
P_D = V_BATT (V_BATT/R_C) + V_BATT (0.012)
+V_OL2 (V_BATT – V_OL2 – V_BE(PNP)) / R_L
= 16(16/236)+16(0.012)+1.5(16–1.5–0.5)/320
= 1.085+0.192+0.066 Watts
= 1.34 Watts
Required board area and Junction Temperature calculation
The maximum junction temperature desired is 150°C. The
permissible temperature rise and required Θ_JA may be calculated
as:
∆T = T_j–T_amb
Θ_JA = ∆T/P_D
Where; ∆T = Temperature rise in °C
P_D = Power dissipation
T_j = Junction Temperature
T_amb = Ambient Temperature
∆T = T_J–T_amb = 150 – 105 = 45°C
Θ_JA = ∆T/P_D=55°C/1.34 watts = 33°C/W.
From Figure 8, the copper area required, using a single sided board,
to keep the junction temperature within limits is approximately
2200 mm². Figure 9 shows 1200 mm² is required on each side of a
double-sided board.
The above example illustrates the worst case situation of the STGD
operating in at a maximum battery voltage, with the lowest nominal
coil resistance (171Ω at room temperature), and at the highest
ambient temperature. This will produce the highest junction
temperature. At lower ambient temperatures the power dissipation
may be higher because the coil resistance is decreased, however
the junction temperature will be lower.
Serial Interface
Figure 10 demonstrates the serial interface timing referenced in the
AC specifications. Figure 11 shows the order of information transfer
through the serial interface. On a low to high transition of the CS pin,
status information replaces the four most significant bits of data in
the shift register and are the first bits shifted out. Output data is
changed on the falling edge of S_CLK, while input data is captured on
the rising edge of S_CLK. Major gauge data is loaded first, starting
with the most significant bit, followed by minor gauge 1 data then
minor gauge 2 data.
S_CLK
t_CSH
t_CF
1
t_CR
29
t_CYC
t_SCLKL
30
t_SCLKH
t_CSL
CS
30 CLOCK CYCLES
DATA_IN
DATA_OUT
D29
t_SU
t_DR
S4
D1
D0*
t_HD
t_DF
D1*
D0*
Figure 10. Serial Interface Timing
SR01499
1998 Apr 03
10