After Sales
Technical Documentation
RAE/RAK–1N
PDA Hardware
Page 6 – 41
Original, 08/96
Table 25. Electrical characteristics (continued)
Notes/conditionsUnitsMax-
mum
Typi-
cal
Mini-
mum
Parameter
VCC5
4.825 5.0 5.175 V tolerance over temperature and
load range
120 225 mV dropout voltage at I=50mA
30 mA regulator I
max
=50mA
peak current 100mA
0.4 1.2 mA ground pin current (supply current)
PWRGOOD
3.04 3.08 3.11 V reset threshold at room temperature
140 240 560 ms power–on reset pulse width
30 ppm/C reset threshold temperature
coefficient
VBACK
2.5 3.0 3.2 V
7 µA nominal load when used
130 mAh capacity, charging not possible
Operating tem-
perature range
–25 +85 °C some circuits tolerate
wider temperature range
Functional Description
VSYS regulator
System voltage regulator is current–mode pulse width modulation (PWM)
dc–dc step–down converter. It has internal switch transistor and current
sense resistor for current–mode control. Oscillator makes the switch
transistor to turn on at fixed frequency. Switch transistor on time is
determined by load taken at output. Current mode feedback makes
transient response fast and provides cycle by cycle current limiting. This
means that outer loop determines threshold voltage for current sense
amplifier to reach to turn switch transistor off. Inner current feedback loop
makes response to load current changes fast.
Outer feedback loop takes output voltage information and compares it to
reference and drives the error towards zero in order to keep regulation.
This kind of control keeps good regulation at output in rapidly changing
load conditions. Tradeoff is high operation current in very light loads
because oscillator is running all the time.
VSYS regulator operates at discontinuous conduction mode (DCM) at light
loads. This means that current through inductor decreases to zero before
new switch transistor turn–on pulse comes and current through inductor
starts to increase. At heavy loads inductor current does not go to zero
and operation is in continuous conduction mode (CCM).
To Next Page
Loading...