Datasheet

TPS783xx
MSP430
10%DutyCycle
GND
EN
IN OUT
EN=Low
NoBattery
SVS
I
LP
BatteryLow
150nA= I
LKG
Low-PowerMode
C
OUT
TPS783xx
MSP430
10%DutyCycle
GND
EN
IN OUT
EN=High
Battery
SVS
I
mC
BatteryOK
500nA
C
OUT
=
t
MAX
V V-
OUT(Nom) MIN
I +I
LKG LP
TPS783xx
SBVS133 –FEBRUARY 2010 www.ti.com
SUPERCAPACITOR-BASED BACKUP
POWER
The very low leakage current at the LDO output gives
a system the flexibility to use the device output
capacitor as a temporary backup power supply for a
short period of time, without the presence of the
battery when the LDO is disabled (during battery
replacement). The leakage current going into the
regulator output from the output capacitor, when the
LDO is disabled, is typically 170nA, see Figure 10.
SYSTEM EXAMPLE
Figure 21. MSP430 Application While Battery is
When the system is active, a voltage supervisor
Replaced
enables the regulator and puts the MSP430 into
active mode when there is a battery installed and its
The time that the capacitor can provide an
voltage is above a certain threshold, as shown in
appropriate voltage level to the MSP430 (that is, the
Figure 20. (The dashed red line indicates the ground
maximum time it should take to replace a depleted
current.)
battery with a new battery), or t
MAX
, can vary from a
few seconds to a few minutes, depending on several
factors, as Equation 1 shows:
• the nominal output of the regulator, V
OUT(Nom)
(equivalent to the initial voltage of the capacitor
when the regulator is disabled);
• the minimum voltage required by the MSP430,
V
MIN
;
• the leakage current into the regulator output, or
I
LKG
;
• the current demand from the MSP430 in
low-power mode, or I
LP
; and
• the size of the output capacitor C
OUT
Figure 20. MSP430 Application in Active Mode
When the battery is depleted, the voltage supervisor
signals the user to replace the system battery. Once
(1)
the battery is removed, the voltage supervisor
disables the regulator and signals the MSP430 to go
The PMOS pass element in the TPS783 series has a
into low-power mode. At this moment, the output
built-in body diode that conducts current when the
capacitor acts as a power supply for the MSP430
voltage at OUT exceeds the voltage at IN. This
during the absence of the battery while it is being
current is not limited, so if extended reverse voltage
replaced, as Figure 21 illustrates. (The dashed red
operation is anticipated, external limiting up to the
line indicates the ground current.)
maximum rated current for the device may be
appropriate.
BOARD LAYOUT RECOMMENDATIONS TO
IMPROVE PSRR AND NOISE PERFORMANCE
To improve ac performance (such as PSRR, output
noise, and transient response), it is recommended
that the printed circuit board (PCB) be designed with
separate ground planes for V
IN
and V
OUT
, with each
ground plane connected only at the GND pin of the
device. In addition, the output capacitor must be as
near to the ground pin of the device as possible to
ensure a common reference for regulation purposes.
High ESR capacitors may degrade PSRR.
10 Copyright © 2010, Texas Instruments Incorporated