Datasheet

LM48511
www.ti.com SNAS416G –JULY 2007–REVISED MAY 2013
SHUTDOWN FUNCTION
The LM48511 features independent amplifier and regulator shutdown controls, allowing each portion of the
device to be disabled or enabled independently. SD_AMP controls the Class D amplifiers, while SD_BOOST
controls the regulator. Driving either inputs low disables the corresponding portion of the device, and reducing
supply current.
When the regulator is disabled, both FB_GND switches open, further reducing shutdown current by eliminating
the current path to GND through the regulator feedback network. Without the GND switches, the feedback
resistors as shown in Figure 1 would consume an additional 165μA from a 5V supply. With the regulator
disabled, there is still a current path from V
DD
, through the inductor and diode, to the amplifier power supply. This
allows the amplifier to operate even when the regulator is disabled. The voltage at PV1 and V1 will be:
(V
DD
- [V
D
+ (I
L
x DCR)]
Where
• V
D
is the forward voltage of the Schottky diode
• V
D
is the forward voltage of the Schottky diode
• I
L
is the current through the inductor
• DCR is the DC resistance of the inductor (2)
Additionally, when the regulator is disabled, an external voltage between 5V and 8V can be applied directly to
PV1 and V1 to power the amplifier.
It is best to switch between ground and V
DD
for minimum current consumption while in shutdown. The LM48511
may be disabled with shutdown voltages in between GND and V
DD
, the idle current will be greater than the
typical 0.1µA value. Increased THD+N may also be observed when a voltage of less than V
DD
is applied to
SD_AMP .
REGULATOR FEEDBACK SELECT
The LM45811 regulator features two feedback paths as shown in Figure 1, which allow the regulator to easily
switch between two different output voltages. The voltage divider consists of the high side resistor, R3, and the
low side resistors (R
LS
), R1 and R2. R3 is connected to the output of the boost regulator, the mid-point of each
divider is connected to FB, and the low side resistors are connected to either FB_GND1 or FB_GND0. FB_SEL
determines which FB_GND switch is closed, which in turn determines which feedback path is used. For example
if FB_SEL = V
DD
, the FB_GND1 switch is closed, while the FB_GND0 switch remains open, creating a current
path through the resistors connected to FB_GND1. Conversely, if FB_SEL = GND, the FB_GND0 switch is
closed, while the FB_GND1 switch remains open, creating a current path through the resistors connected to
FB_GND0.
FB_SEL can be susceptible to noise interference. To prevent an accidental state change, either bypass FB_SEL
with a 0.1µF capacitor to GND, or connect the higher voltage feedback network to FB_GND0, and the lower
voltage feedback network to FB_GND1. Because the higher output voltage configuration typically generates
more noise on V
DD
, this configuration minimizes the V
DD
noise exposure of FB_SEL, as FB_SEL = GND for
FB_GND0 (high voltage output) and FB_SEL = V
DD
for FB_GND1 (low voltage output).
The selectable feedback networks maximize efficiency in two ways. In applications where the system power
supply voltage changes, such as a mobile GPS receiver, that transitions from battery power, to AC line, to a car
power adapter, the LM48511 can be configured to generate a lower voltage when the system power supply
voltages is lower, and conversely, generate a higher voltage when the system power supply is higher. See the
Setting the Regulator Output Voltage (PV1) section.
In applications where the same speaker/amplifier combination is used for different purposes with different audio
power requirements, such as a cell phone ear piece/speaker phone speaker, the ability to quickly switch between
two different voltages allows for optimization of the amplifier power supply, increasing overall system efficiency.
When audio power demands are low (ear piece mode) the regulator output voltage can be set lower, reducing
quiescent current consumption. When audio power demands increase (speaker phone mode), a higher voltage
increases the amplifier headroom, increasing the audio power delivered to the speaker.
Copyright © 2007–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM48511