ADM7170ACPZ-R2 datasheet(17/24 Pages) AD

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ADM7170

Data Sheet

Rev. C | Page 16 of 23

THEORY OF OPERATION

The ADM7170 is a low quiescent current, low dropout linear

regulator that operates from 2.3 V to 6.5 V and provides up to

500 mA of load current. Drawing a low 3.0 mA of quiescent

current (typical) at full load makes the ADM7170 ideal for

portable equipment. Typical shutdown current consumption is

0.25 μA at room temperature.

Optimized for use with small 4.7 µF ceramic capacitors, the

ADM7170 provides excellent transient performance.

Figure 54. Internal Block Diagram

Internally, the ADM7170 consists of a reference, an error

amplifier, a feedback voltage divider, and a PMOS pass

transistor. Output current is delivered via the PMOS pass

device, which is controlled by the error amplifier. The error

amplifier compares the reference voltage with the feedback

voltage from the output and amplifies the difference. When the

feedback voltage is lower than the reference voltage, the gate

of the PMOS device is pulled lower, allowing more current to

pass and increasing the output voltage. When the feedback

voltage is higher than the reference voltage, the gate of the

PMOS device is pulled higher, allowing less current to pass and

decreasing the output voltage.

The ADM7170 is available in 17 fixed output voltage options,

ranging from 1.2 V to 5 V. The ADM7170 architecture allows

any fixed output voltage to be set to a higher voltage with an

external voltage divider. For example, a fixed 5 V output

ADM7170 can be set to a 6 V output according to the following

equation:

VOUT = 5 V(1 + R1/R2)

Figure 55. Typical Adjustable Output Voltage Application Schematic

Use a value of less than 200 kΩ for R2 to minimize errors in the

output voltage caused by the SENSE pin input current. For

example, when R1 and R2 each equal 200 kΩ and the default

output voltage is 1.2 V, the adjusted output voltage is 2.4 V. The

output voltage error introduced by the SENSE pin input current is

0.1 mV or 0.004%, assuming a typical SENSE pin input bias

current of 1 nA at 25°C.

The ADM7170 uses the EN pin to enable and disable the VOUT

pins under normal operating conditions. When EN is high, VOUT

turns on; when EN is low, VOUT turns off. For automatic startup,

tie EN to VIN (Pin 7 or Pin 8).

VOUT

SENSE

GND

CURRENT-LIMIT,

THERMAL

PROTECT

SOFT START

REFERENCE

SHUTDOWN

VIN

VOUT

SENSE

VIN

ADM7170

GND

VIN

VOUT

CSS

1nF

CIN

4.7µF

COUT

4.7µF

OFF

VIN = 6.5V

VOUT = 6.0V

2kΩ

10kΩ

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