14
MC13181 Advance Information
MOTOROLA
Circuit Features
3.2 Reference Regulator
The Reference Regulator supplies a precise 1.265 V for use by the other on-chip regulators. To maintain
spectral purity on the LDO regulators, this internal reference is not intended for external loading. A pin is
provided for an off-chip 0.47
μ
F capacitor for bypassing.
The reference regulator derives its power from the VCC1 pin. Since the reference regulator supplies
reference to the other regulators, power should not be applied to V
CC2
if V
CC1
is unpowered.
3.3 Shutdown
The active-low shutdown input disables all regulators and logic. In the shutdown state, the total IC current
consumption is 2.0
μ
A. Shutdown also disconnects the input resistor of the divider on the VIN- input to the
comparator. As this would typically be connected to V
CC
for battery voltage detection. Shutting down
MC13181 removes this approximately 5.0
μ
A current.
Care should be taken if this pin is to be driven from a processor operating on a voltage supplied by
MC13181.
3.4 Reset Circuit
The RESET_B output goes high after a delay, based on the delay capacitor. It is initiated when V
LDO_1.85
rises above the reset threshold of 1.70 V. When V
LDO_1.85
falls below the reset threshold, RESET_B goes
low with no delay.
To calculate the value of the delay capacitor needed for a given delay, the formula C/I = dV/dT can be
used. At the start of the delay, the capacitor is shorted nearly to 0 V, and then charged with a 500 nA
current source until its voltage reaches the reference voltage, 1.265 V.
T delay = C * V / I, simplifies to
T delay (ms) ~= 2.53 x C (nF)
or,
C (nF) ~= T / 2.53 (T in ms)
Note: The threshold voltage, resistance of the shorting FET and the current source will vary with
temperature and V
CC2
, so the resulting delay will vary by 2:1 or more.
3.5 Comparator with Programmable Hysteresis
The MC13181 includes a general purpose comparator. The IC's VIN- input incorporates internal resistors
scaling (by a factor of approximately 0.44) the voltage to the minus input of the comparator. The IC's
VIN+ input is directly connected to the plus input of the comparator. A typical application connects the
VIN- input to V
CC2
and VIN+ to the reference output for Under-Voltage Detection. The scaling resistors
thereby set the threshold to 2.88 V with no additional components required.
Hysteresis, of typically 30 mV, can be enabled or disabled via the HYSSEL pin (low is enable), allowing
versatility for general purpose applications.
The comparator may be used for general purpose applications, if the fixed divider ratio is accounted for.
The inputs should not go lower than 1.0 V. The plus input should not exceed V
CC2
- 1.0 V and the minus
input should not exceed V
CC2
.
The output of the comparator is push-pull, referenced to V
LDO_1.85
.