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LOW DROPOUT VOLTAGE REGULATOR WITH HIGH STABILITY OUTPUT CURRENT LIMIT CIRCUIT

IP.com Disclosure Number: IPCOM000008821D
Original Publication Date: 2002-Jul-16
Included in the Prior Art Database: 2002-Jul-16
Document File: 4 page(s) / 61K

Publishing Venue

Motorola

Related People

Gary Pace: INVENTOR [+2]

Abstract

A cellular phone handset requires a low dropout voltage regulator with a current limit function to prevent the IC from being destroyed under heavy load conditions or when the output is suddenly shorted to ground. The current limit circuit must not significantly affect the load regulation or stability of the regulator during normal operation, especially when the load current approaches the current limit. This paper describes a low dropout voltage regulator with a current limit feedback control loop that reacts quickly to protect the regulator from damage during overload conditions. The current limit circuit has minimal effect on regulator load regulation and provides high regulator stability prior to and following the application of excessive load currents.

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Low Dropout Voltage Regulator with High Stability

Output Current Limit Circuit

Gary Pace and Kai Zhong

ABSTRACT

A cellular phone handset requires a low dropout voltage regulator with a current limit function to prevent the IC from being destroyed under heavy load conditions or when the output is suddenly shorted to ground. The current limit circuit must not significantly affect the load regulation or stability of the regulator during normal operation, especially when the load current approaches the current limit.  This paper describes a low dropout voltage regulator with a current limit feedback control loop that reacts quickly to protect the regulator from damage during overload conditions.  The current limit circuit has minimal effect on regulator load regulation and provides high regulator stability prior to and following the application of excessive load currents.

PROBLEM DESCRIPTION

The power dissipation in a cellular phone can become excessive and damage the product if a high current load or short is applied to one of the power supplies during manufacture, repair or during normal operation.  There are normally multiple linear regulators supplying power in a phone that need to be protected from this type of overload damage, and this must be done in a manner that does not degrade regulator performance during normal phone operation.  What is required is a protection circuit that senses the regulator output current and lowers the output voltage as necessary to prevent the output current from exceeding a predetermined value.

SOLUTION

Figure 1 is a block diagram of the low dropout voltage regulator with current limit function that protects the circuit from excessive current loads. In this circuit a current limit feedback loop gain control transistor Q2 has been added in series with the output drive transistor Q1 in the voltage feedback loop to form a cascode configuration. The remainder of the voltage feedback loop is composed of the regulator pass transistor M1, the resistive voltage divider R3 and R4, and the error amplifier. A reference voltage Vref is applied to the error amplifier. The reference voltage Vref and the ratio of R3 to R4 set the output voltage of the regulator as is well known in the art. A transconductance amplifier Gm1 with a predetermined input offset voltage Vo receives a voltage input representative of the regulator output load current flowing through the regulator pass transistor M1 and generates an output current into the base of the gain control transistor Q2 and into a voltage clamp circuit Vclamp1 when the transconductance amplifier input voltage is less than the predetermined input offset voltage Vo. The Vclamp circuit voltage is selected to bias the transistor Q1 collect...