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LM2753 Datasheet(PDF) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2753
Description  High Power Switched Capacitor Voltage Convertor/Flash LED Driver
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LM2753 Datasheet(HTML) 6 Page - National Semiconductor (TI)

   
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Application Information (Continued)
PFM REGULATION
The LM2753 achieves its tightly regulated output voltage
with pulse-frequency modulated (PFM) regulation. PFM sim-
ply means the part only pumps when charge needs to be
delivered to the output in order to keep the output voltage in
regulation. When the output voltage is above the target
regulation voltage the part idles, consuming minimal supply
current with C
1 is connected between VIN and GND and VIN
is disconnected from V
OUT. In this state, the load current is
supplied solely by the charge stored on the output capacitor.
As this capacitor discharges and the output voltage falls
below the target regulation voltage, the charge pump acti-
vates, and charge is delivered to the output. This charge
supplies the load current and boosts the voltage on the
output capacitor.
The primary benefit of PFM regulation is when output cur-
rents are light and the part is predominantly in the low-
supply-current idle state. Net supply current is minimal be-
cause the part only occasionally needs to recharge the
output capacitor by activating the charge pump. With PFM
regulation, input and output ripple frequencies vary signifi-
cantly, and are dependent on output current, input voltage,
and to a lesser degree, other factors such as temperature,
internal switch characteristics, and capacitor characteristics
(voltage tolerance, temperature variation).
OUTPUT VOLTAGE RIPPLE
The voltage ripple on the output of the LM2753 is highly
dependent on the application conditions. The output capaci-
tance, input voltage, and output current each play a signifi-
cant part in determining the output voltage ripple. Due to the
complexity of the LM2753 operation, providing equations or
models to approximate the magnitude of the ripple cannot be
easily accomplished. However, the following general state-
ments can be made.
The output capacitor will have a significant effect on output
voltage ripple magnitude. Ripple magnitude will typically be
linearly proportional to the output capacitance present. The
ESR of the output capacitor also contributes to the output
voltage ripple, as there is effectively an AC voltage drop
across the ESR due to current switching in and out of the
capacitor. To keep the voltage ripple small, a low-ESR ce-
ramic capacitor is recommended on the output. Placing mul-
tiple capacitors in parallel can reduce ripple significantly, by
both increasing capacitance and reducing ESR. When ca-
pacitors are in parallel the ESR of the capacitors are in
parallel as well, resulting in a net ESR according to the
properties of parallel resistance. Two identical capacitors in
parallel have twice the capacitance and half the ESR as
compared to a single capacitor if the same type. On a similar
note, if a large-value, high-ESR capacitor (tantalum, for ex-
ample) is to be used as the primary output capacitor, the net
ESR can be significantly reduced by placing a low-ESR
ceramic capacitor in parallel with this primary output capaci-
tor.
I
OUT PIN
An internal FET is connected between the V
OUT pin and the
I
OUT pin of the LM2753. When a logic high signal is placed
on the Flash input pin, the internal FET turns on and con-
nects I
OUT to VOUT in less than 10ns (typ). If the IOUT pin is
not going to be used, the Flash input pin can be tied to GND
and the I
OUT pin can be left unconnected.
In the typical application circuit there is one resistor between
V
OUT and IOUT and another resistor between IOUT and the
Flash LED. When a LOW logic signal is placed on the Flash
input pin, the internal FET opens and current flows from
V
OUT through both resistors and through the Flash LED.
When a logic HIGH signal is applied to the Flash input pin
the internal FET closes, shorting out the resistor between
V
OUT and IOUT, and current flows through the second resis-
tor and the Flash LED.
Follow the steps below to set the desired current levels for
the Flash LED:
Setting Flash Current
1.
Determine the LED’s forward voltage at the desired
Flash current.
2.
Find the voltage difference between I
OUT and the LED
forward voltage.
3.
Divide the voltage difference by the desired Flash cur-
rent to obtain the needed Flash LED ballast resistance
Setting Torch Current
1.
First determine required Flash Ballast
2.
Determine the LED’s forward voltage at the desired
continuous Torch current
3.
Find the voltage difference between V
OUT and the LED
forward voltage.
4.
Divide the voltage difference by the desired Torch cur-
rent to obtain the total resistance needed.
5.
Subtract the Flash Ballast resistance from this total re-
sistance to find the required Torch resistance between
V
OUT and IOUT
PWM BRIGHTNESS CONTROL PROCEDURES
The brightness of a Flash LED connected to V
OUT can be
linearly varied from zero up to the maximum programmed
current level by applying a Pulse-Width-Modulated signal to
the EN pin of the LM2753. The following procedures illus-
trate how to program the LED drive current and adjust the
output current level using a PWM signal.
1.
To select the maximum desired current level, refer to the
"I
OUT Pin" section and follow the steps detailed in the
"Setting Flash Current" and "Setting Torch Current" sub-
headings.
2.
Brightness control for "Torch" mode can be implemented
by pulsing a signal at the EN pin, while Flash is con-
nected to a logic LOW signal. Also, brightness control
can also be implemented for Flash mode by pulsing a
signal on the Flash pin while the part is already enabled
(EN = logic HIGH). LED brightness is proportional to the
duty cycle (D) of the PWM signal. For linear brightness
control over the full duty cycle adjustment range, the
PWM frequency (f) should be limited during Torch mode
to accommodate the turn-on time (T
ON = 640µs) of the
device. Also, the PWM frequency should be limited dur-
ing "Flash" mode to accommodate the turn-on time
(T
FLASH = 10ns) of the IOUT output FET.
D x (1/f) > T
ON,FLASH
f
MAX =DMIN ÷TON,FLASH
If the PWM frequency is much less than 100Hz, flicker
may be seen in the LEDs. For the LM2753, zero duty
cycle will turn off the LED and a 50% duty cycle will
result in an average I
OUT being half of the programmed
LED current. For example, if the output is programmed
for a maximum of 100mA through the Flash LED, a 50%
duty cycle will result in an average I
LED of 50mA.
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