Philips Semiconductors Product data
SA57017-XX
CapFREE 150 mA, low-noise, low-dropout
regulator with thermal protection
2003 Aug 27 8
TECHNICAL DISCUSSION
The SA57017 is a low drop-out, low-quiescent current linear
regulator designed primarily for battery-powered applications and
stabilizes with or without input/output capacitors. The device delivers
up to 150 mA and is available with preset output voltages of 1.25 V,
1.6 V, 1.8 V, 2.0 V, 2.2 V, 2.4 V, and 2.6 V, 2.8 V, 3.0 V, and 3.3 V for
both SOT 23-5 and WL-CSP packages.
The 1.25 band-gap reference is connected to the error amplifier’s
inverting input. The error amplifier compares this reference with the
feedback voltage and amplifies the difference. If the feedback
voltage is lower than the reference voltage, the pass-transistor gate
is pulled lower, which allows more current to pass to the output. The
output voltage is fed back through an internal resistor voltage divider
connected to the Vout pin.
Band-gap
The band-gap circuitry generates a temperature independent
voltage by properly adding two voltages with negative and positive
temperature coef ficient. The band-gap voltage is typically 1.25 volts
with a temperature variation of 5 mV over the temperature range
from –40 °C to 125 °C.
Low Pass Filter
Low pass filter is basically an RC filter with a low cutoff frequency.
No external capacitor is used. There is one comparator, which
turns on the bypass paths to charge or discharge the capacitor if the
output of the filter is higher or lower than the band-gap voltage by a
specified amount.
Output Amplifier
The output amplifier is a folded cascode PMOS amplifier which
controls the gate of the output transistor and sources the load
current. A portion of the output voltage is compared to the reference
voltage and a constant voltage is maintained at output. The output is
also monitored by a comparator which trips the PWROK if the output
voltage falls below the nominal output level by a specified amount
due to low battery condition or any other reason. The current limiter
circuit monitors the output current and limits the load current to a
certain value to avoid any damage due to short circuit.
Bias Circuit
The bias block provides bias currents and voltages for the other
blocks. It has a self start-up circuit and it can establish the bias
currents and voltages very fast.
Temperature Sensor
The temperature sensor block monitors the die temperature and
flags Power OK when the temperature crosses 133 °C. If the die
temperature goes beyond 165 °C typical value the output amplifier is
shut down. Both the temperatures corresponding to PWROK or
PWRON have a hysteresis of 10 °C.
Reverse Battery Protection
The reverse battery protection circuit prevents damage to the device
if the supply battery is accidentally installed backwards. This circuit
compares VIN and PWRON to ground and disconnects the device’s
internal circuits if it detects reversed polarity. Reverse supply current
is limited to 1mA when this protective circuit is active, preventing the
battery from rapidly discharging through the device. Reverse
battery current is extremely low, 0.5 mA typical.
PWROK Output
PWROK goes low when the output voltage goes out of regulation as
during drop-out, current limit or thermal shutdown. PowerOK is an
open-drain N-channel MOSFET. To obtain a logic-level output,
connect a 10 kΩ pull-up resistor from PWROK pin to VOUT pin. To
minimize current consumption, make this resistor as large as
practical. A 100 kΩ resistor works well for most applications. The
PowerOK is not active during shutdown. PWROK has a 10 µs
blanking time to allow transient responses to settle within accuracy
limits.
Current Limit
The SA57017 includes a current limiter that monitors and controls
the pass transistor’s gate voltage, limiting the output current to
300 mA typical value. For design purposes, consider the current
limit to be 160 mA minimum value. The output can be shorted to
ground for an indefinite period of time without damaging the part.