Datasheet

ENA
D1A
D2A
D1B
D2B
ENB
I
SET
GND
TPS7510x
V
IN
DimmingPWM
orCPUGPIO
R
SET
(optional)
Li-Ion
Battery
TPS7510x
SBVS080I –SEPTEMBER 2006–REVISED NOVEMBER 2013 www.ti.com
USE OF ENABLE PINS FOR PWM DIMMING LOAD REGULATION
The TPS7510x divides control of the LED outputs into The TPS7510x is designed to provide very tight load
two banks of two current sources each. Each bank is regulation. In the case of a fixed current source, the
controlled by the use of an independent, active-high output load change is a change in voltage. Tight load
enable pin (ENA and ENB). The enable pin can be regulation means that output voltages (LED forward
used for standard ON/OFF operation of the current voltages) with large variations can be used without
source, driven by standard logic levels from impacting the fixed current being sourced by the
processor GPIO pins, for example. Drive EN high to output or the output-to-output current matching. The
turn on the bank of LEDs; drive EN low to turn off the permissible variation on the output not only allows for
bank of LEDs. large variations in white LED forward voltages, but
even permits the use of different color LEDs on
Another use of the enable pin is for LED dimming.
different outputs with minimal effect on output current.
LED brightness is a function of the current level being
driven across the diode and the time that current is
LINE REGULATION
being driven through the diode. The perceived
brightness of an LED can be changed by either
The TPS7510x is also designed to provide very tight
varying the current level or, more effectively, by
line regulation. This architecture allows for voltage
changing the time in which that current is present.
transient events to occur on the power supply
When a PWM signal is input into the enable pin, the
(battery) without impacting the fixed output current
duty cycle (high or ON time) determines how long the
levels or the output to output current matching. A
fixed current is driven across the LEDs. Reducing or
prime example of such a supply transient event is the
increasing that duration has the effect of dimming or
occurrence of a transmit pulse on the radio of a
brightening the LED, without having to employ the
mobile handset. These transient pulses can cause
more complex method of varying the current level.
variations of 300mV and 600mV on the supply to the
This technique is particularly useful for reducing LED
TPS7510x. The line regulation limitation is that the
brightness in low ambient light conditions, where LED
lower supply voltage level of the event does not
brightness is not required, thereby decreasing current
cause the input to output voltage difference to drop
consumption. The enable pins can also be used for
below the dropout voltage range.
LED blinking, varying blink rates based on system
status.
Although providing many useful applications, PWM
dimming does have a minimum duty cycle required to
achieve the required current level. The recommended
minimum on time of the TPS7510x is approximately
33μs. On times less than 33μs result in reductions in
the output current by not allowing enough time for the
output to reach the desired current level. Also, having
both enables switching together, asynchronously, or
having one enable on at all times, impacts the
minimum recommended on time (see Figure 4 and
Figure 5). If one enable is already on, the speed at
which the other channel turns on is faster than if both
channel were turning on together or if the other
channel is off. Therefore, connecting one enable on
Figure 13. Typical Application Diagram
allows for approximately 10μs to 12μs shorter
minimum on times of the switching enable channel.
Unused enable pins can be left unconnected or
connected to ground to minimize current
consumption. Connecting unused enable pins to
ground increases ESD protection. If connected to V
IN
,
a small amount of current drains through the enable
input (see the Electrical Characteristics table).
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