Circuit Note

CN-0523

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Rev. 0 | 4 of 8

Figure 9. CN0523 Stability Factor and Measure vs. Frequency

OVERTEMPERATURE MANAGEMENT

An overtemperature management feature is implemented on the

CN0523 wherein the amplifiers are automatically disabled when

the board temperature reaches a preset threshold. The CN0523

automatically returns to normal operation once the temperature

falls below the hysteresis set point. This feature is done using

the open-drain output (TOVER/TUNDER) function of the ADT6401

temperature switch, which monitors the temperature near the first

stage HMC407 amplifier and compares it to a pin-programmable

trip point.

Figure 10 shows the trip point monitoring circuitry using the

ADT6401 temperature switch.

Figure 10. ADT6401 Block Diagram

The states of Pins S0, S1, and S2 select the temperature trip point

and hysteresis of the ADT6401. The available temperature trip point

and hysteresis settings on the CN0523 are listed in Table 1.

Table 1. Selecting the ADT6401 Trip Point and Hysteresis

S0

S1

S2

Temperature Trip

Point

Hysteresis

0

1

Float

0

0

0

0

0

0

+45°C

+55°C

+65°C

2°C

2°C

2°C

0

1

Float

1

1

1

0

0

0

+75°C

+85°C

+95°C

2°C

2°C

2°C

0

1

Float

Float

Float

Float

0

0

0

+105°C

+115°C

+55°C

2°C

2°C

10°C

0

1

Float

0

0

0

0

1

1

1

1

1

+65°C

+75°C

+85°C

+95°C

10°C

10°C

10°C

10°C

By default, the CN0523 reference design uses a +95°C trip point

and a +10°C hysteresis setting.

The HMC407 has an internal power-down capability that can be

directly controlled using the ADT6401 output, as shown in Figure

11.

Figure 11. CN0523 Overtemperature Management Circuit

LAYOUT CONSIDERATIONS

The power amplifier produces a large amount of heat when in use;

therefore, special attention must be given to the thermal dissipation.

To ease the power dissipation, the CN0523 uses a 3-layer thick

ground plane and multiple thermal vias were placed around and

under the HMC407.

Figure 12 is a thermal camera image of the CN0523 operating with

an input power of 0 dBm. At an ambient temperature of 17°C, the

temperature at the surface of the HMC407 is approximately 80°C

(a 63°C temperature rise). The ADT6401 sensor is located between

the HMC407s and rises 47.7°C (above ambient) to 64.7°C, thus the

HMC407's temperature rise is 1.321 times than that of the sensor.

Under these operating conditions, the ADT6401 reaches its trip

point of 95°C at an ambient temperature of 47°C. With the HMC407

at 47°C ambient + 63°C rise, the trip point is at 110°C – well

below the 150°C maximum die temperature. The trip point can be

adjusted to suit other conditions, taking care to observe the 150°C

maximum.