AMIS-30421 Datasheet, PDF (25/41 Pages) ON Semiconductor – Micro-Stepping Stepper Motor Bridge Controller

AMIS−30421
POWER SUPPLY AND THERMAL CALCULATION
Logic Supply Regulator
AMIS−30421 has an on−chip 3.3V low−drop regulator to
supply the digital part of the chip itself, some low−voltage
analog blocks and external circuitry. See Table 4 for the
limitations.
Over− and Undervoltage
AMIS−30421 has undervoltage detection. If V_BB drops
below V_BBUL, the drivers are disabled. To be able to enable
the drivers again the V_BB voltage needs to rise above
V_BBUH.
Overvoltage detection is also present. If the voltage rises
above V_BBOH the drivers are disabled. The voltage needs to
drop below V_BBOL to be able to enable the driver again. See
also Figure 5.
Start−Up Behavior
Figure 4 gives the start−up of AMIS−30421. After V_BB is
applied and after a certain power up time (t_PU), the internal
voltage regulator V_DD will start−up. When V_DD gets above
V_DDH, the internal POR will be released and the digital will
start−up. The WDb−pin will be kept low for an additional
100ms (t_POR). After the WDb−pin is deactivated and after a
time t_DSPI, SPI communication can be initiated.
Junction Temperature Calculation
To calculate the junction temperature of AMIS−30421 the
thermal resistance junction−to−ambient must be known.
When only a PCB heat sink is used, a typical value is
30°C/W (see Table 4).
There are three modes the junction temperature can be
calculated for.
• In Sleep Mode (<SLP> = ‘1’) the V_BAT consumption is
maximum 150 mA making T_j = T_amb.
• In Normal Mode when the driver is disabled
(<MOTEN> = ‘0’), the V_BAT consumption is maximum
20 mA (no external load on VDD−pin). The junction
temperature can be calculated as next:
T_J + T_A ) ^ǒV_BAT I_BAT Rth_JA^Ǔ
For an 18 V application operating at an ambient
temperature of 125°C this would give:
T_J + 125° C ) ^ǒ18 V 20 mA 30° CńW^Ǔ
T_J + 135.8° C
• In Normal Mode with the driver enabled (<MOTEN> =
‘1’) the gate charge current needs to be included in the
calculations.
I_BAT + 20 mA ) ^ǒ6 V_REGH C_ISS f_PWM^Ǔ
For an 18 V application driving external MOSFET’s with
an input capacitance of 1 nF this would result in:
I_BAT + 20 mA ) ^ǒ6 12.8 V 1 nF 30 kHz^Ǔ
I_BAT + 22.3 mA
Operating at 125°C ambient temperature this result in a
junction temperature of:
T_J + 125° C ) ^ǒ18 V 22.3 mA 30° CńW^Ǔ
T_J + 137° C
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