Datasheet MAX8887, MAX8888 (Maxim) - 6

Low-Dropout, 300mA Linear Regulators in SOT23 Detailed Description
when the output voltage is below 93% of the nominal value to provide foldback current limiting. The MAX8887/MAX8888 are low-dropout, low-quies- cent-current linear regulators designed primarily for
Thermal Overload Protection
battery-powered applications. The devices supply Thermal overload protection limits total power dissipa- loads up to 300mA and are available in several fixed tion in the MAX8887/MAX8888. When the junction tem- output voltages in the 1.5V to 3.3V range. The perature exceeds TJ =+170°C, a thermal sensor turns MAX8887 is optimized for low-noise operation, while off the pass transistor, allowing the device to cool. The the MAX8888 includes an open-drain POK output flag. thermal sensor turns the pass transistor on again after As illustrated in Figure 1, the MAX8888 consists of a the junction temperature cools by 20°C, resulting in a 1.25V reference, error amplifier, p-channel pass tran- pulsed output during continuous thermal overload con- sistor, and internal feedback voltage divider. ditions. Thermal overload protection protects the MAX8887/MAX8888 in the event of fault conditions. For
Internal p-Channel Pass Transistor
continuous operation, do not exceed the absolute maxi- The MAX8887/MAX8888 feature a 0.5Ω p-channel mum junction-temperature rating of T MOSFET pass transistor. Unlike similar designs using J=+150°C. PNP pass transistors, p-channel MOSFETs require no
Operating Region and Power Dissipation
base drive, which reduces quiescent current. PNP- The MAX8887/MAX8888’s maximum power dissipation based regulators also waste considerable current in depends on the thermal resistance of the IC package dropout when the pass transistor saturates and uses and circuit board. The temperature difference between
MAX8887/MAX8888
high base drive currents under large loads. The the die junction and ambient air, and the rate of air flow. MAX8887/MAX8888 do not suffer from these problems The power dissipated in the device is P = IOUT ✕ (VIN- and consume only 55µA of quiescent current under VOUT). The maximum allowed power dissipation is heavy loads as well as in dropout. 727mW or:
Output Voltage Selection
PMAX = (TJ(MAX) - TA) / (θJC + θCA) The MAX8887/MAX8888 are supplied with various facto- where TJ(MAX)-TA is the temperature difference ry-set output voltages ranging from 1.5V to 3.3V. The between the MAX8887/MAX8888 die junction and the part number’s two-digit suffix identifies the nominal out- surrounding air; θJC is the thermal resistance from the put voltage. For example, the MAX8887EZK33 has a pre- junction to the case; and θCA is the thermal resistance set output voltage of 3.3V (see the Ordering Information). from the case through PC board, copper traces, and other materials to the surrounding air.
Shutdown
Drive SHDN low to enter shutdown. During shutdown, Refer to Figure 2 for the MAX8887/MAX888 valid oper- the output is disconnected from the input and supply ating region. current drops to 0.1µA. When in shutdown, POK and
Noise Reduction
OUT are driven low. SHDN can be pulled as high as For the MAX8887 only, an external 0.01µF bypass 6V, regardless of the input and output voltages. capacitor at BP creates a lowpass filter for noise reduc-
Power-OK Output
tion. The MAX8887 exhibits 42µVRMS of output voltage The power-OK output (POK) pulls low when OUT is less noise with CBP = 0.01µF and COUT = 2.2µF (see the than 93% of the nominal regulation voltage. Once OUT Typical Operating Characteristics). exceeds 93% of the nominal voltage, POK goes high
Applications Information
impedance. POK is an open-drain n-channel output. To obtain a logic level output, connect a pullup resistor
Capacitor Selection and Regulator
from POK to OUT. A 100kΩ resistor works well for most
Stability
applications. POK can be used as a power-on-reset Connect a 2.2µF ceramic capacitor between IN and (POR) signal to a microcontroller (µC) or to drive other ground and a 2.2µF ceramic capacitor between OUT logic. Adding a capacitor from POK to ground creates and ground. The input capacitor (CIN) lowers the POK delay. When the MAX8887 is shut down, POK is source impedance of the input supply. Reduce noise held low independent of the output voltage. If unused, and improve load-transient response, stability, and leave POK grounded or unconnected. power-supply rejection by using a larger ceramic out- put capacitor such as 4.7µF.
Current Limit
The MAX8887/MAX8888 monitor and control the pass The output capacitor’s (COUT) equivalent series resis- transistor’s gate voltage, limiting the output current to tance (ESR) affects stability and output noise. Use out- 0.8A (typ). This current limit is reduced to 500mA (typ) put capacitors with an ESR of 0.1Ω or less to ensure
6 _______________________________________________________________________________________