IRF7401 datasheet(20/24 Pages) AD

20 / 24 page

REV. C

–20–

ADN8830

If the voltage at VSY drops below VX, Q1 is turned off and the

VLIM pin will be set to 1.5 V, effectively setting the maximum

voltage across the outputs to 0 V. The voltage divider for VX is

calculated from Equation 43.

Design Example 5

A maximum output current limit needs to be set at 1.5 A for a

TEC with a maximum voltage rating of 2.5 V. The ADN8830 is

powered from 5 V. The TEC resistance is estimated at 1.67

Ω and

efficiency at 85%. Using Equation 43, the voltage drop across

RS will be 8.8 mV when 1.5 A is delivered to the TEC. The trip

voltage VX is set to 4.991 V with R3 = 178

Ω and R4 = 100 kΩ

as shown in Figure 21. To set the output voltage limit to 2.5 V,

the voltage at VLIMIT should be set to 0.875 V according to

Equation 17.

The C1 capacitor is added to smooth the voltage transitions at

VLIM. Once an overcurrent condition is detected, the output

voltage will turn down to 0 V within 30 ms.

For a more exact measurement of the output current, place a

sense resistor in series with the output load, as shown in Figure 23.

The AD626 instrumentation amplifier is set for a gain of 100

with a reference voltage of 2.47 V from VREF. The output of

the AD626 is equal to 100

× R

× I

L and is fed to the AD8602,

which is set up as a window comparator. With VX greater than

VLO but less than VHI, VLIM will be pulled down to the volt-

age at VLIMIT. Should VX fall outside the voltage window,

VLIM will be pulled to 1.5 V as in Figure 21. The trip points

should be set according to

VHI

VREF

R I

VLO

VREF

R I

S LIMIT

=×

100

–

(44)

AVDD

Q1,Q2

FDG6303N

EQUIVA-

LENT

VREF

1nF

AVDD

OUT B

10m

TEC

VLIMIT

(0V TO 1.5V)

VLIM

1.47k

3.48k

AVDD

200k

300k

200k

VLO

AD8602

AD626

AVDD

VHI

Figure 23. High Accuracy Output Current Limit

The upper and lower trip point voltages can be set independently,

allowing different maximum output current limits depending on

the direction of the current. The resistor divider for VHI and

VLO is tapped to VREF to maintain window accuracy with any

changes in VREF. Using the values from Figure 23 with a 5 V

supply, the output current will not exceed 1.5 A in either direction.

Adding the current sensing resistor will slightly reduce efficiency.

The power dissipated by this resistor is D

× ITEC2 × R

S if the

TEC is heating, or (1–D)

× ITEC2 × R

S if the TEC is cooling.

Include this when calculating efficiency as described in the

Calculating Power Dissipation and Efficiency section.

25mV/ C

OUTOA

ADT70

OUTIA

–INOA

5.11k

AGND

4.99k

RGA

RGB

INST

AMP

–INIA

+INIA

+INOA

82.5

IOUTB

IOUTA

RTD

82.5

GND

SENSE

NOTE: ADDITIONAL PINS OMITTED FOR CLARITY

TO THERM_IN

= 1V @ 25 C

Figure 22. Using an RTD for Temperature Feedback to the ADN8830

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