### AXP Translator Whitepaper

NXP’s AXP translators
Introduction
In order to reduce power dissipation in electronic applications there is a trend toward lower
supply voltages. A consequence of this trend is a necessity to interface in modular designs
between applications at different supply voltages.
NXP has added innovative voltage level translating devices to its existing Advanced eXtremely
low Power (AXP) logic family to further improve its portfolio of translator products. These
translating gates ensure interface compatibility of new lower voltage designs to existing
products without serious power dissipation penalties.
Requirements of low power logic families
As general-purpose components, logic devices are used at different frequencies and power
supply voltages in a multitude of applications. This large diversity has produced the need
to express a single parameter that can be used in determining the power dissipation of a
device in a given application. Looking at the components of power dissipation, we can better
understand the evolution that has occurred within discrete logic to reduce it.
1. Quiescent power dissipation
Theoretically, when a CMOS device is not switching and the input levels are GND or VCC, no
direct MOS transistor current path exists between VCC and GND. In practice, however, thermally
generated minority carriers allow a very small leakage current to flow between VCC and GND. This
leakage current is typically a few nano-ampere, and produces the quiescent supply current ICC, as
it is represented in device datasheets. To calculate maximum quiescent power dissipation use:
PDQ = VCC x ICC(1)
2. Static power dissipation
In the case where full swing input levels (GND or VCC) cannot be applied, a direct MOS
transistor current path can exist between VCC and GND, which leads to additional supply
current through the input buffer and results in additional power dissipation.Δ∆ICC represents this
additional current due to an input level of VCC – 0.6 V. To calculate the static power dissipation
use:
PDSTAT = VCC x (ICC + n∆ICC)
(2)
Where n represents the number of inputs held at VCC – 0.6 V.
3. Dynamic power dissipation
When clocking a CMOS device, the charging and discharging of on-chip parasitic and load
capacitance dissipates power. Further power dissipation occurs at the point the output
switches, when both the p-channel and the n-channel transistors are partially conducting. This
transient energy loss is typically only 10% of that due to parasitic capacitance. Due to the linear
relationship between power dissipation and frequency in CMOS devices, the model of the
device for power dissipation calculations is a capacitance (CPD)
The total dynamic power dissipation per device is:
PDDYN = ∑(CPDVCC2 fI) + ∑(CLVCC2 fO) (3)
where:
CPDis the power dissipation capacitance per buffer
fI is the input frequency
fO is the output frequency
CL is the total external load capacitance per output.
4. Total power dissipation
Combining the components of static and dynamic power dissipation results in an equation for
the total power dissipation:
PD = VCC x (ICC + n∆ICC) + ∑(CPDVCC2 fI) + ∑(CLVCC2 fO)(4)
Assuming that the frequency of an application is constant, it becomes clear from equation
four that to reduce total system power dissipation, system designers need to lower the supply
voltage VCC as much as they can, and where possible drive all digital inputs with VCC or GND.
The discrete logic supplier must offer logic families that provide the required speed
performance at the lower voltages, have the lowest power dissipation capacitance (CPD)
possible and have inputs that present the lowest possible capacitive load (CL).
In 2005, NXP released the Advanced Ultra-low Power CMOS (AUP) logic family. AUP is the
lowest power logic family suitable for 3.3 V applications; it is fully specified for applications
from 1.1–3.6 V, allowing the migration of many applications from the 3.3V node to 1.8V and
beyond.
In 2012 NXP logic released the Advanced eXtremely low Power CMOS (AXP) logic family.
AXP is the lowest power logic family suitable for 2.5 V applications; it is fully specified for
applications from 0.75–2.75 V, allowing the migration of many applications from the 1.8V node
to 1.2V and beyond. It provides the lowest propagation delay of all low power families, and
critically, it provides system designers with the lowest CPD, typically just 2.9 pF, which is more
than a 50% reduction when compared to competing low voltage / low power families.
Another power saving feature is the low capacitance of AXP inputs. The capacitive load seen
by any device driving an AXP input is up to 40% less than when driving other low voltage
families, leading to a dynamic power dissipation savings at the system level. Recognizing that it
is not always possible to drive inputs at VCC and GND, NXP has also designed the AXP inputs to
have very low ∆ICC.
NXP’s AXP translators
2
When driving a push-pull logic input, a long output transition will lead to higher cross bar
current through the input, and thus increased system power dissipation. To avoid this situation,
the output transition time must be reduced. However, doing so in an uncontrolled manner can
lead to high dV/dt events that result in reflections, signal integrity issues and in some cases
EMC issues. Unlike other low voltage families that utilize low current drive outputs, AUP and
AXP outputs incorporate edge rate feedback to ensure minimal power dissipation through the
driven input buffer without jeopardizing signal integrity.
AXP translators—supporting modular designs
From a time-to-market perspective, it is often necessary to re-use previously tested and
qualified circuits instead of completely redeveloping them. In modular designs, this may limit
the supply voltage options available to the designer. Although it is possible to develop a newer
circuit at 1.8 V, a requirement may be to interface to existing solutions at 3.3 V or 5.0 V. A result
of this situation could be decisions to:
•continue to use the higher voltage with its 83% (3.3 V) or 178% (5.0 V) higher power
dissipation penalty
•use a supply voltage level compatible with the switching levels of the 3.3 V existing
solution and accept a higher dissipation due to ∆ICC.
Neither of these approaches are attractive from a power dissipation standpoint. To enable low
power designs that are compatible with existing modular solutions, NXP has introduced its
portfolio of AXP translating gates and buffers. These low power dual supply devices combine
the 0.7 – 2.75 V AXP input stage with a higher voltage, low noise 1.2 – 5.5 V output stage.
Inputs
The AXP translators use the existing AXP inputs. These are fully specified for supply voltage
ranges of 2.3 – 2.7 V, 1.65 – 1.95 V, 1.4 – 1.6 V, 1.1 – 1.3 V and 0.75 – 0.85 V. The ESD protection
circuit used results in the input being over voltage tolerant to 2.75 V. This tolerance permits the
application of input signals that exceed the supply voltage. The input options include Schmitttrigger inputs and Schmitt-trigger action inputs. Schmitt-trigger action makes the input
tolerant of slower input transition rates. Hysteresis is not specified, but the input can tolerate
input transition rise and fall rates of 200 ns/V. Schmitt-trigger inputs include an input hysteresis
specification and have no restriction on input transition rates. Table 1 shows the typical
characteristics of the Schmitt-trigger action input; Table 2 shows the typical characteristics of
the Schmitt-trigger input.
NXP’s AXP translators
3
Table 1. Typical Schmitt-trigger action input characteristics of AXP Translators
VCC = 2.5 V
VCC = 1.8 V
aaa-010193
0.8
VCC = 1.5 V
aaa-010192
280
ICC
(µA)
ICC
(µA)
0
70
140
0.4
0
0
0.5
1.0
1.5
2.0
VI (V)
2.5
0
VCC = 1.2 V
0.5
1.0
1.5
VI (V)
0
2.0
0
0.5
1.0
VI (V)
1.5
VCC = 0.8 V
aaa-010190
60
aaa-010191
140
ICC
(µA)
aaa-010189
2
ICC
(µA)
ICC
(µA)
1
30
0
0
0.5
1.0
VI (V)
0
1.5
0
0.5
VI (V)
1.0
Table 2. Typical Schmitt-trigger input characteristics of AXP Translators
VCC = 2.5 V
VCC = 1.8 V
aaa-010203
1.2
ICC
(µA)
VCC = 1.5 V
aaa-010202
500
ICC
(µA)
0.6
0
250
0
0.5
1.0
1.5
2.0
VI (V)
0
2.5
150
0
VCC = 1.2 V
0.5
1.0
1.5
VI (V)
2.0
0
0
0.5
1.0
VI (V)
1.5
VCC = 0.8 V
aaa-010200
100
aaa-010201
300
ICC
(µA)
ICC
(µA)
aaa-010199
3
ICC
(µA)
2
50
1
0
0
0.5
1.0
NXP’s AXP translators
VI (V)
1.5
0
0
0.5
VI (V)
1.0
4
The use of AXP inputs facilitates interfacing to lower voltage outputs without the excessive ∆ICC
due to mismatch between the input HIGH logic level and supply voltage.
Outputs
AXP translators incorporate a newly developed output structure to enable interfacing to higher
voltage inputs. The output is fully specified for supply voltage ranges of 4.5–5.5 V, 3.0–3.6 V,
2.3–2.7 V, 1.65–1.95 V and 1.4–1.6 V. To support partial power down mode, the output features
IOFF, which ensures there is no current leakage path through the outputs when the device
supply voltage is set to 0 V. Table 3 shows the output characteristics of AXP translators.
Table 3. Typical output characteristics of AXP Translators
VCC = 5.0 V
VCC = 3.3 V
VCC = 2.5 V
VCC = 1.8 V
VCC = 1.5 V
VCC = 1.2 V
NXP’s AXP translators
5
Low power dissipation and input capacitance
The use of NXP’s state-of-the-art C050 process ensures that AXP translators have the lowest
parasitic capacitance possible. This approach results in a typical input capacitance of 0.5 pF,
or 50% of that of leading AVC 3 V translator solutions, and 25% of leading LVC 5 V translator
solutions. The effect on the power dissipation capacitance is equally as impressive. AXP
translators are modelled as a 0.8 pF capacitance on the input supply (CPDI) and a 7.6 pF
capacitance on the output supply (CPDO). By comparison, existing leading 3 V solutions have CPDI
and CPDO of 2 pF and 17 pF, respectively; existing leading 5 V solutions have CPDI and CPDO of 4 pF
and 18 pF, respectively. These values are used with equation 5 to estimate power dissipation.
PDDYN = ∑(CPDIVCCI2 fI) + ∑((CL+CPDO)VCCO2 fO)(5)
Table 4 shows a comparison of dynamic power dissipation of leading translator families.
Table 4. Comparison of power dissipation capacitance of leading translator families
LVC
VCC
CPDI
AVC
CPDO
CPDI
AXP
CPDO
CPDI
CPDO
0.8 V
—
—
1
9
0.5
—
1.2 V
—
—
2
11
0.6
6.9
1.5 V
—
—
2
11
0.7
6.9
1.8 V
2
15
2
12
0.8
6.9
2.5 V
3
16
2
14
1.0
7.0
3.3 V
3
16
2
17
—
7.2
5.0 V
4
18
—
—
—
7.6
Low noise characteristics
Typically, at a supply voltage of 0.7 V, the input switching threshold will be 350 mV. As a result,
when translating between high and low voltage domains, it is important to consider the effect
of high voltage output noise on switching of the low voltage input. Figure 1 shows the noise on
an output set LOW when another output transitions LOW to HIGH and HIGH to LOW.
NXP’s AXP translators
6
VCCI = 0.7 V; VCCO = 5.5 V; LOW to HIGH transition
350
300
250
VOL (mV)
200
150
100
50
0
-50
-100
0
50
100
Time (ns)
150
200
VCCI = 0.7 V; VCCO = 5.5 V; HIGH to LOW transition
350
300
250
VOL (mV)
200
150
100
50
0
-50
-100
0
50
100
Time (ns)
150
200
Figure 1 Noise characteristics of AXP Translators
VOL(p) is the maximum value of VOL. It represents the change in the GND level due to the output
switching. It is important to minimize this value, as the GND level is the reference point for the
input switching level. It can be seen that the worst case VOL(p) for AXP translators is 50 mV, much
less than the > 350 mV required to switch the input.
Reduced power and footprint
AXP translators are available in industry standard leaded PicoGate and leadless MicroPak
packages, making them suitable for volume constrained (area and height) portable applications
such as smart phones and tablet PCs.
The lower gate count Mini logic products reduce time-to-market by making it easy to
implement last-minute changes. They also improve the cost-effectiveness of crowded layouts,
by simplifying routing and eliminating dependencies in intricate line-layout patterns. Table 2
shows the AXP translator packages currently available.
Table 5. Package options for AXP translating gates
Package
suffix
GW
GX
GW
GM
GN
GS
DC
5-pin
5-pin
6-pin
6-pin
6-pin
6-pin
8-pin
Package
SOT353
SOT1226
SOT363
SOT886
SOT1115
SOT1202
SOT765-1
Width (mm)
2.10
0.80
2.10
1.00
1.00
1.00
3.10
Length (mm)
2.00
0.80
2.00
1.45
0.90
1.00
2.00
Height (mm)
1.00
0.35
1.00
0.50
0.35
0.35
1.00
Pitch (mm)
0.65
0.50
0.65
0.50
0.30
0.35
0.50
Package
suffix
GT
GN
GS
DP
GU
GF
8-pin
8-pin
8-pin
10-pin
10-pin
10-pin
Package
SOT833
SOT1116
SOT1203
SOT552-1
SOT1160-1
SOT1081-2
1.00
1.00
1.00
3.00
1.40
1.00
Width (mm)
Length (mm)
1.95
1.20
1.35
3.00
1.80
1.70
Height (mm)
0.50
0.35
0.35
1.10
0.50
0.50
Pitch (mm)
0.50
0.30
0.35
0.50
0.40
0.35
Summary
By providing low-power voltage level translator products, NXP Semiconductors continues to
support the migration of applications to lower voltages, resulting in power savings. In modular
designs, newer applications can take advantage of the lower power provided by reducing the
supply voltage without having to redesign existing higher voltage applications. AXP translators
provide the interface: they are the lowest dynamic and static power translators in the industry.
Inputs and outputs support partial power-down applications by not presenting a current
leakage path when the device is powered down. Low noise outputs ensure signal integrity. AXP
translators enable voltage level translation between an input range of 0.7 V to 2.75 V and an
output supply range of 1.2 V to 5.5 V. They are available in leading industry packages.