Using the AD5755 family in Applications Without Dynamic Power Control

Document created by DRice Employee on Nov 28, 2013Last modified by DRice Employee on Jan 27, 2014
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For applications that do not require Dynamic Power Control it is possible to set up the AD5755 family of products without the use of the DC-to-DC converter. This reduces the number of external components required and is useful for low power applications with space constraints which require the AD5755 quad-channel feature.   This document describes two alternatives to using the DC-to-DC converter. 

 

Method 1 – External PMOS:

As an alternative to the DC-to-DC converter, an external PMOS transistor can be used to limit the on-chip power dissipation, though this will not reduce the power dissipation of the total system.  On the AD5755 / AD5735 and AD5755-1, the PMOS circuit is set up as shown in Figure 1. 

Leave the SWx pin open-circuit.   Tie AVCC to DVDD.  This is to maintain the voltage on the AVCC pin above the most negative supply (AVSS or 0 V (only in AD5755-1, AD5737 and AD5757)).   If the voltage on the AVCC pin is equal to or below the most negative voltage, it can result in latch-up. See table 1 for recommended supplies for the AVCC and VBOOST rails.  All other pins are set up in the same manner as when the Dynamic Power Control function is in use. Details are outlined in the Pin Configuration and Layout Guidelines sections of the AD5755 datasheet.

 

 

Figure 1.bmp

Figure 1. Configuration of a Channel using an External Zener Diode

VBOOST is powered externally and the zener diode holds the gate of the external PMOS at (VBOOST – Zener Voltage).   This means that the majority of the channel’s power dissipation will take place in the external PMOS transistor.   The power dissipated by the PMOS is calculated as follows (using worst case figures):

 

VBOOST = 33 V

Zener Voltage = 5 V

RLOAD = 0Ω, IOUT = 24 mA

One Channel:

On-Chip Power: 5 V x 0.024 A = 0.12 W

Off-Chip Power: 28V x 0.024A = 0.672 W

Four Channels:

On-Chip Power = 0.12 W x 4 = 0.48 W

Off-Chip Power = 0.672 W x 4 = 2.688W

 

It is important when selecting R1 that the power is kept low. In the example above, R1 = 1MΩ and 33 V is the voltage supplied.   There is a 5V drop across the zener diode. This means that there is 28uA flowing through the 1MΩ resistor (0.784mW dissipated).

 

Figure 2 shows the AD5757/AD5737 which has a pin (IGATE) dedicated to controlling an external PMOS, therefore a zener diode is not required.  The IGATE pin is only used when Dynamic Power Control feature is not being used. It holds the gate of the external PMOS at VBOOST – 5V causing the majority of the power dissipation to take place on the external PMOS. Leave the SWx pin open-circuit.  

 

Tie AVCC to DVDD.  This is to maintain the voltage on the AVCC pin above the most negative supply (AVSS or 0 V (only in AD5755-1, AD5737 and AD5757)).   If the voltage on the AVCC pin is equal to or below the most negative voltage, it can result in latch-up. See table 1 for recommended supplies for the AVCC and VBOOST rails.  All other pins are set up in the same manner as when the Dynamic Power Control function is in use. Details are outlined in the Pin Configuration and Layout Guidelines sections of the AD5755 datasheet.

 

Figure 2.bmp

Figure 2: Configuration of a Channel using IGATE (AD5757 or AD5737)


In both cases, the PMOS chosen must be able to tolerate a VDS voltage of –VBOOST and handle the power dissipation required.  The PMOS will typically have minimal effect on current output performance.   

 

Method 2 – Connect VBOOST To External Supply or AVDD:

VBOOST must be powered for both voltage and current output ranges,  VBOOST can be tied to AVDD   as long as sufficient headroom is maintained between VBOOST and the output per the formulae in table 1.  Otherwise supply VBOOST with its own power supply.

 

 

Figure 3 shows the AD5755 setup without Dynamic Power Control.  Leave the SWx pin open-circuit.   Tie AVCC to DVDD.  This is to maintain the voltage on the AVCC pin above the most negative supply (AVSS or 0 V (only in AD5755-1, AD5737 and AD5757)).   If the voltage on the AVCC pin is equal to or below the most negative voltage, it can result in latch-up. See table 1 for recommended supplies for the AVCC and VBOOST rails.  All other pins are set up in the same manner as when the Dynamic Power Control function is in use. Details are outlined in the Pin Configuration and Layout Guidelines sections of the AD5755 datasheet.

 

Figure 3.bmp

Figure 3: AD5755 without Dynamic Power Control on One Channel

Please note that if dynamic power control is not used then it is of even greater importance to understand the effects of power dissipation.   The AD5755 is packaged in a 64-lead, 9mm x 9mm LFCSP package.   The thermal impedance, θJA, is 28°C/W.  It is important that the device is not operated under conditions that exceed the junction temperature limit (125OC). Worst-case conditions occur when the AD5755 is at maximum VBOOST (33V) and driving the maximum current (24mA) to ground (RLOAD = 0Ω). The quiescent current of the AD5755 must also be taken into account.


The calculations in table 2 estimate maximum power dissipation under these worst-case conditions, and determine maximum ambient temperature based on this. These figures assume that proper layout and grounding techniques are followed to minimize power dissipation, as outlined in the Layout Guidelines section of the AD5755 datasheet.  It also refers to the operating currents as specified in the AD5755 datasheet.

 

Table 1. Changes in Power Supplies

 

PowerRecommended Minimum/Maximum Voltage
AVCCDVDD to 5.5V
VBOOST

7.5 V (Min) to 33 V (Max)

Current Output: (IOUT × RLOAD) + Headroom (Typically 2.4 V headroom, maximum 2.7 V headroom)
Voltage Output: Typically 15 V or Maximum VOUT + Headroom (Maximum 2.2 V headroom)


 

 

Table 2. Thermal and Supply Conditions (Assuming AVSS = - 15 V, AVCC = 5 V, AVDD/VBOOST = 33V)

 

Considerations:Maximum Rating:

Maximum allowed power dissipation when operating at an ambient

  temperature of 85°C

Eqn1.PNG

Maximum allowed ambient temperature – One Channel

AVDD: 33V x 0.0075A = 0.2475W

AVSS: -15V x 0.0017A = 0.0255W

AVCC: 5 V x 0.001A = 0.005W

VBOOST (1 Channel):  33V x 0.025A = 0.825W

Total (1 Channel): 1.103W

Temperature Increase: 1.103 x 28 = 30.9OC

Max Ambient Temperature: 125OC – 30.9 OC = 94.1OC

Maximum allowed ambient temperature – Four Channels

AVDD: 33V x 0.0075A = 0.2475W

AVSS: -15V x 0.0017A = 0.0255W

AVCC: 5 V x 0.001A = 0.005W

VBOOST (4 Channels): (33 V x 0.025 A) x 4 = 3.3W

Total (4 Channels): 3.578W

Temperature Increase: 3.578 x 28 = 100.18OC

Max Ambient Temperature: 125 OC – 100.18 OC = 24.8OC


Please visit the product page for more information about the AD5755

AD5755 datasheet and product info | Quad Channel, 16-Bit, Serial Input, 4 mA to 20 mA and Voltage...

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