Abstract

MAX14919/MAX14919A are 4-channel, low-side Digital Output devices. Each Individual output channel can support up to 500mA of output current. This application note provides necessary information to consider when output channels are paralleled to achieve increased output current in a typical digital output application.

Introduction

The MAX14919/MAX14919A are industrial-protected, 4-channel, low-side switches commonly used in industrial solutions. The digital output devices driving loads/ actuators must operate in harsh environments as the loads are present in the field (industry floors, field equipment controllers, and remote open areas). The digital output devices must handle Electrostatic Discharge (ESD), surge, and inductive load demagnetization events with or without external components and operate reliably.

The MAX14919/MAX14919A device can be powered from an available 5V supply or a typical 24VDC field supply. Integrated clamps present at each output of the MAX14919/MAX14919A provide up to ±1kV/42O surge protection and inductive energy clamping at 55V (typical). A low 140mO channel on-resistance enables low power dissipation and reduced self-heating. The integration and smaller form factor enable MAX14919/ MAX14919A to be used for cost saving and high-density applications

Figure 1 shows a typical application example of an isolated quad, low-side digital output (DO) application using the MAX14919/MAX14919A.

For industrial DO applications that require more than 500mA load current per channel for switching actuators and loads, multiple channels may be connected in parallel to increase the output capability. In Figure 2, all the four channels are connected to increase channel current to 2A (typical).

This application may be sufficient when driving a purely resistive load. However, a typical industrial actuator is inductive in nature, and the switch must safely discharge the energy in the inductor when the switch opens. In this case, external TVS protection must be applied at the output of the low-side DO switch as the voltage across the switch is increased as is the energy that needs to be discharged during an inductive demagnetization. Failure to adhere to device specifications and normal operating limits as mentioned in the MAX14919/MAX14919A data sheet results in damage to the device.

Inductive Demagnetization in the MAX14919

Figure 3 shows when the channel output switch is turned off, the back EMF or counter electromotive force generated across the inductor due to the change in current flow creates a high voltage spike at the switch output. When this transient voltage spike crosses the clamp voltage of MAX14919 reaches 55V typical, the integrated clamp is activated, turning on the internal FET to discharge the induced energy across the load. The internal switch is turned on in the high resistance mode, resulting in self-heating of the pass device. This can be observed in the FAULT status output, indicating an overtemperature warning.

The Energy present in the load inductance is given by:

where L is the inductance of the load (H) and I_LOAD is the load current (A).

The total energy dissipated in the MAX14919 internal clamp during an inductive load turn-off event can be estimated by:

V_CLAMP is the clamping voltage at the output of the MAX14919 due to the internal integrated clamps. I_LOAD is the initial current flowing through the inductive load before turn-off.

V_FIELD is the available system power/ field supply to the load.

During active clamping, the time taken by the switch to dissipate the energy in equation 2 is:

The MAX14919/ MAX14919A can simultaneously discharge the inductive energy across a 1.2H inductance in series with a 48O resistive load at each channel with a 500mA load current. When dissipating the energy through the device, if the local temperature at the output switch increases beyond thermal warning or shutdown temperature, the status indicating output, FAULT, transitions low during the load turn-off period. Figure 4a shows the MAX14919/MAX14919A output response when only one channel is subjected to demagnetization. Figure 4b shows the output response when all four channels are subjected to demagnetization simultaneously.

The clamping energy, E_CLAMP, absorbed by the MAX14919 should not exceed 200mJ per channel at 125°C. Any increase at each channel can result in damage to the device due to excessive thermal stress. Generally, when switching load currents beyond 500mA with load inductance larger in the 1.2H range, for single-channel or multichannel applications, add an external TVS diode clamp that can safely absorb the energy.

Single-Channel, 2A Low-Side DO Application with MAX14919/ MAX14919A

Industrial digital output modules/controllers commonly have either 500mA per channel or 2A nominal per channel output specifications. The following section covers design considerations when paralleling the outputs to increase the current output to 2A (Figure 5).

The design engineer must consider the following items to implement and optimize as per the design requirements to achieve a robust and reliable operation of a digital output device for temperatures up to 85°C.

1. Maximum Output Current (2.4A)

Designing for 20% tolerance of 24V DC field supply to go as much as 28.8V, the application must handle up to 2.4A of output current. Short circuit and overcurrent conditions can be handled by using the MAX14919's R_SET resistor that protects the outputs by limiting the output current to a set value. See the MAX14919/MAX14919A data sheet for different R_SET values for different levels of current limiting. The R_SET of 31.05kO is used to set the current limit at 600mA minimum per channel.

The four output switches in the chip are well-matched within +/-1% typical on-resistance matching at the same local junction silicon temperatures. A MOSFET's on-resistance has a positive thermal temperature coefficient in triode operation. This has the positive side-effect that if a channel has slightly lower-on-resistance, it carries higher current when connected in parallel to another channel, which increases its junction temperature until a level that the on-resistances are matched, which results in equal current sharing between the channels connected in parallel. Adhering to good layout techniques and using low impedance wider traces for output channel interconnects in the design helps in minimizing I²R losses.

Selecting the External Clamp in 2A DO application in the MAX14919

Figure 5 shows a single-channel, 2A low-side DO application that operates over the -40°C to 85°C temperature range. The external TVS diode aids in inductive energy discharge and surge protection. A maximum of 1.2H load inductance is considered when demagnetization occurs.

External TVS diode selection is vital to ensure robust operation of the low-side switch in the application. If the TVS diode is chosen correctly, then all the energy dissipation is through this external clamp. Other than switching and steady-state load currents, the DO is not subjected to electrical stress and operates normally at a given temperature.

Design for Margin

This section explains the procedure to pick the appropriate TVS clamp for a 2A low-side DO solution. The following parameters are considered for the analysis, referring to the Figure 4 application diagram:

V_FIELD-MAX = 28.8V, L = 1.2H, R = 12O, 33V_RM TVS Selection: SMCJ33A (Littelfuse),

I_LOAD-MAX = 2.4A, P_{PK_DIS-MAX} = V_CLAMP at I_LOAD-MAX x I_LOAD-MAX (W)

1. TVS Reverse Working /Standoff Voltage (V_RM)

   The TVS reverse working voltage (sometimes called standoff voltage) is the low leakage current (1uA or similar) voltage range that should be chosen larger than the maximum rated field power supply voltage to not to cause high leakage current currents in the switch-off state. A 33V working voltage is commonly chosen as it is higher than the 28V max supply voltage.

2. TVS Clamping Voltage During Surge

   The maximum clamping voltage of the TVS diode comes into consideration for surge protection as surge currents are much larger than normal load currents. If surge protection is required, the maximum clamping voltage of the external TVS diode at the peak surge current level must be lower (if the surge energy is to be absorbed by the external clamp) than the clamping voltage of the internal clamp during the surge pulse. The internal surge clamping voltage of a MAX14919A output for +1kV/42? surge is higher than +60V and for negative surges is clamped by the body diode at -1V level.

3. TVS Clamping Voltage

   The clamping voltage can be estimated for the given ambient temperature and current level by the two-step process:

1. When the clamping event is non-repetitive or spaced by a large period, sufficient for the junction temperature of the clamp to settle to ambient conditions with negligible or low value clamping test currents (for example I_T = 1mA), the clamping voltage change with ambient temperature can be determined by:

   

   where V_CL is the clamping voltage and is equal to the breakdown voltage value available from the diode data sheet. aT is the temperature coefficient (/°C). T_J is the junction temperature of the diode that settles to the ambient temperature value.

   Table 1. Clamping Voltage of SMCJ33 Over Temperature

   Ambient Temperature	Estimation	Observation
   TA = 25°C	38.4V	38.1V
   TA = 85°C	40.4V	40V

2. The current through the diode during inductive kickback affects the clamping voltage.

   The effective clamping voltage at the ambient temperature for a given load current can be determined by:

   

   The dynamic resistance can be determined from R_D vs. pulse duration curves in the TVS diode data sheets. Alternatively, it can be calculated by measuring the slope of change in the clamping voltage to different current levels ¹. See Table 2 for the variation of clamping voltage with different current levels at T_A = 25°C and T_A = 85°C. The dynamic resistor R_D computed from observations of different clamping voltages with current levels is applied to equation 6. The effective clamping voltage (V_CLeff) calculated must be approximately equal to the observed clamping voltage levels as shown in Table 2.

   The maximum current level (2.4A) during clamping in the application is less compared to the peak current levels that the diode can handle. Hence, the dynamic resistance is approximated to be the same for the given current levels and temperature.

   Table 2. Effective Clamping Voltage Over Current

   Ambient Temperature	ILOAD	VCLAMP (V)Observation	VCLAMP (V) Estimation*
   TA = 25°C	100mA	39	38.34
   	500mA	40	39.3
   	2.15A	44	43.26
   TA = 85°C	100mA	41	40.626
   	500mA	42	41.59
   	2.15A	45	45.55
   *(From equation 5 and 6), RD Calculated = 2.4O [SMCJ33A (Littelfuse)]

Peak Power Dissipation Consideration for the Given Demagnetization Pulse

The peak power dissipation characteristics found in TVS diode data sheets are based on the surge/test signal that has exponential current decay characteristics defined in the diode data sheets as shown in Figure 6. The t_d is the time taken for the current to reach half of its peak value.

The typical operating curves 'Peak Pulse Power vs. Pulse width (t_d)' and 'Peak Pulse Power Derating in % vs. Initial Junction Temperature' provide the power dissipation capability of the diode at elevated temperatures. For reliable and robust operation, the peak power dissipation of the diode computed from Table 2 for the application described in Figure 4 must be within the derated line at its maximum operating temperature (T_A = +85°C).

The shape of a typical current decay behavior during the demagnetization event can be approximated to a sawtooth or a triangular waveform (see Figure 7 and Figure 8). Estimate the t_d for this curve by determining the pulse duration equivalence factor for the same power dissipation for different signal types, before referring to the peak power dissipation, temperature derating curves. ²

For a given demagnetization event (triangular signal), the time taken to reach half of the peak current (IPPM/2) is ~40% of the total demagnetization time for the current to reach zero or the clamping to be deactivated. This 40% ensures to keep margins into consideration. Table 3 shows the observed and estimated values of peak power dissipation and demagnetization times.

Figures 9a through 9e show the comparison of the peak power dissipation curves of various TVS diodes (obtained from diode data sheet) in SMC (1.5kW), SMC (3kW), SMD (5kW) types, and the peak power dissipation (P_{PK_DIS-MAX}) when discharging 2.4A of kickback current. The data beyond 10ms is extrapolated for analysis. The dotted lines are the derated power dissipation lines at 85°C ambient temperatures and derating percentage is dependent on the manufacturer. The performance of generic 1.5kW SMC TVS diode from different manufacturers are compared along with special 3kW and 5kW devices also included in the comparison.

The peak dissipation from the TVS diode during the demagnetization event needs to be under the dotted line (devices 85°C derated value) to maintain safe operation. A typical evaluation of the application with the diodes listed in Table 4 yielded a successful demagnetization event over the temperature range without any damage to the diode or the MAX14919A chip.

It can be concurred that 1.5kW, 3kW, and 5kW TVS options are within the safer operation region to the derated line at hot, in the logarithmic scale. The higher power dissipation rating devices (3kW, 5kW) allow more headroom or margin in the design. The above procedure is a typical evaluation, and the designer must be careful in selection to ensure reliable operation with the design for margin and volume.

Steady-State Power Dissipation Considerations

What if the load is repetitively subjected to turning on and off? This means the TVS clamp must de-energize the load due to inductive demagnetization continuously over a period.

In the application, consider the output inductive load is switched at a given frequency continuously and the average power dissipation across the diode must be taken into consideration as demagnetization occurs at every falling edge of the cycle. Because of the repetitive nature of the event, the average junction temperature of the diode increases as the frequency of switching (f_s) increases. The average junction temperature of the diode must not exceed the absolute maximum rating of the diode.

Because of the bigger package size of the surface mount diodes, these components have recommendations on the soldering footprint that defines the mounting method. And hence, the power dissipation can dissipate better from junction to lead rather than junction to case. The junction temperature (T_J) can be determined by the average power and the device's junction to lead thermal resistance as shown in equations 7 and 8, and must be below the maximum operation junction temperature for a given switching rate (f_s).

Alternatively, the direct method is to compare the average power obtained by equation 7 and the steady-state derating or the steady-state power dissipation curve provided in the TVS diode manufacturer's data sheet.

Conclusion

The MAX14919/MAX14919A can be operated up to 2A low-side DO application by paralleling its output channels with necessary considerations to choose an external clamp for robust and reliable operation.

References/Other Resources

¹ Calculation of Transil Apparent Dynamic Resistance - ST life.augmented

² TVS Clamping Protection Mode - ST life.augmented

Trademarks

• Littelfuse is a registered trademark of Littelfuse Inc.
• Bourns is a registered trademark of Bourns, Inc.
• ST Micro is a registered trademark of ST Microelectronics, Inc.