General Description

The MAX1735 negative-output, low-dropout linear regulator operates from a -2.5V to -6.5V input and delivers a guaranteed 200mA with a low 80mV dropout. The high accuracy (±1%) output voltage is preset or can be adjusted from -1.25V to -5.5V with an external resistive voltage-divider.

An internal N-channel MOSFET allows for a low 85µA quiescent current virtually independent of the load,making this device ideal for battery-powered portable equipment, such as PDAs, mobile phones, cordless phones, and wireless data modems.

The device is available in several preset output voltage versions: -5.0V, -3.0V, and -2.5V. All versions offer a 1nA low-power shutdown mode, short-circuit protection, and thermal overload protection. The device is offered in a tiny 5-pin SOT23 package.

 

Applications

Disk Drives

Modems

Instrumentation Amplifiers

Notebook Computers

Mobile and Cordless Telephones

PCMCIA Cards

GaAsFET Bias

Mobile Wireless Data Modems

PDAs and Palmtop Computers

 

Features

Guaranteed 200mA Output Current

Low 80mV Dropout Voltage at 200mA

Low 85µA Quiescent Supply Current

Low 1nA Current Shutdown Mode

Stable with 1µF COUT

PSRR >60dB at 100Hz

Thermal Overload Protection

Short-Circuit Protection-5.0V, -3.0V, or -2.5V Output Voltage or Adjustable (-1.25V to -5.5V)

Tiny SOT23-5 Package

 

Detailed Description

The MAX1735 is a low-dropout negative linear voltage regulator. It features Dual Mode operation, allowing a fixed -5.0V, -3.0V, or -2.5V output voltage or an adjustable output from -1.25V to -5.5V. The regulator is guaranteed to supply 200mA of output current. It features 60dB power-supply rejection for noise-sensitive applications and a low 85µA operating current that optimizes it for battery-operated devices.

As Figure 1 illustrates, the device consists of an internal-1.25V reference, an error amplifier, an N-channel MOSFET, an internal precision-trimmed feedback voltage-divider, and a Dual Mode comparator.

The -1.25V reference is connected to the inverting input of the error amplifier. The error amplifier compares the reference voltage with the selected feedback voltage and amplifies the difference. The error amplifier drives the MOSFET to control the output voltage.

The feedback voltage for regulation is generated by either an internal or external resistive voltage-divider connected from OUT to SET. The internal Dual Mode comparator selects the feedback path based on VSET.Connect SET to GND to use the internal feedback path,setting the output voltage to the preset value. If an external voltage-divider is used, see Output Voltage Selection.

 

Internal N-Channel MOSFET

The MAX1735 features an N-channel MOSFET pass transistor. Unlike similar designs using NPN bipolar pass transistors, N-channel MOSFETs require extremely low drive currents, reducing overall quiescent current. Also, NPN-based regulators consume still more base current in dropout conditions when the pass transistor saturates. The MAX1735 does not suffer from these problems, consuming only 125µA total current at full load and in dropout.

 

Output Voltage Selection

The MAX1735 features Dual Mode operation, allowing for a preset or adjustable output voltage. In preset voltage mode, the output of the MAX1735 is set to -5.0V, -3.0V, or-2.5V. Select this mode by connecting SET to GND.

 

Current Limiting

The MAX1735 features a current limit that protects the regulator. Short-circuit output current is typically 515mA. The output will withstand a short to ground indefinitely; however, if the increased power dissipation heats the die to +160°C, the thermal overload protection will shut off the regulator, preventing damage to the IC.

 

Applications Information

Capacitor Selection and Regulator Stability

Capacitors are required at the input and output of the MAX1735. Connect a 1µF or greater capacitor between IN and GND. This input capacitor serves only to lower the source impedance of the input supply in transient conditions; a smaller value can be used when the regulator is powered from a low-impedance source, such as another regulated supply or low-impedance batteries. For output voltages between -2.5V and -5.5V, connect a 1µF or greater capacitor between OUT and GND. For voltages between -1.25V and -2.5V, use a 2.2µF or greater output capacitor. The maximum value of the output capacitor to guarantee stability is 10µF.

The output capacitor’s value and equivalent series resistance (ESR) affect stability and output noise. To ensure stability and optimum transient response, output capacitor ESR should be 0.1Ω or less for output voltages from -1.25V to -2.45V and 0.2Ω or less for output voltages between -2.5V and -5.5V. Inexpensive surface-mount ceramic capacitors typically have very-low ESR and are commonly available in values up to 10µF.Other low-ESR capacitors, such as surface-mount tantalum, may also be used. Do not use low-cost aluminum electrolytic capacitors due to their large size and relatively high ESR. Lastly, make sure the input and output capacitors are as close to the IC as possible to minimize the impact of PC board trace impedance.