LM2751SD-B/NOPB National Semiconductor, LM2751SD-B/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM2751SD-B/NOPB
• Description: IC LED DRIVR WHITE BCKLGT 10-LLP
• Manufacturer: National Semiconductor
• Type: Backlight, White LEDr

Specifications of LM2751SD-B/NOPB

• Topology: PWM, Switched Capacitor (Charge Pump)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 725kHz
• Voltage - Supply: 2.8 V ~ 5.5 V
• Voltage - Output: 4.5V
• Mounting Type: Surface Mount
• Package / Case: 10-LLP
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 150mA
• Internal Switch(s): Yes
• Efficiency: 90%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM2751SD-B; LM2751SD-BTR

Application Information
CIRCUIT DESCRIPTION
The LM2751 is a Switched Capacitor Convertor with gains of
2x and 1.5x. It is capable of continuously supplying up to
150mA at 4.5V or up to 80mA at 5V depending on the output
voltage option. The LM2751’s fixed frequency pre-regulation
maintains the output voltage to within 3% (typ.), making it
well suited for driving White LEDs. There are also four user
programmable switching frequencies to reduce the quies-
cent current consumption at light loads.
Aside from powering LEDs, the LM2751 is suitable for driv-
ing other devices with power requirements up to 150mA. The
LM2751 operates over the extended Li-Ion battery range
from 2.8V to 5.5V. The LM2751 limits output current to
250mA (typ.) during an output short circuit condition. LED
brightness is controlled by applying a PWM (Pulse Width
Modulation) signal to the Enable pin (EN). (see PWM
BRIGHTNESS CONTROL section).
SOFT START
Soft Start is engaged when the device is taken out of Shut-
down mode (EN = logic HIGH) or when voltage is supplied
simultaneously to the V
voltage on V
reference voltage is being ramped up. The output voltage is
programmed to rise from 0V to the regulated output voltage
level (4.5V or 5V) in 300µs (typ.).
ENABLE MODE
The Enable logic pin (EN) disables the part and reduces the
quiescent current to 0.77µA (typ.). The LM2751 has an
active-high enable pin (LOW = shut down, HIGH = operat-
ing) which can be driven with a low-voltage CMOS logic
signal (1.5V logic, 1.8V logic, etc). There is an internal 1MΩ
pull-down resistor between the EN and GND pins of the
LM2751.
FREQUENCY MODE SELECT
The LM2751 switching frequency is user programmable via
two logic input pins, CS0 and CS1. Both logic input pins have
active-high logic (LOW = un-selected, HIGH = selected) and
can be driven with a low-voltage CMOS logic signal (1.5V
logic, 1.8V logic, etc). There are no internal pull-down or
pull-up resistors between the CSx and GND pins of the
LM2751. The CSO and CS1 can be controlled independently
or with the same logic signal.
The selectable switching frequencies are 9.5kHz, 37kHz,
300kHz, 725kHz. The switching frequency is programmed
according to Table 1
V
The LM2751 uses pre-regulation to regulate the output volt-
age to 4.5V or 5.0V depending on the voltage option. Pre-
regulation uses the voltage present at V
OUT
CS0
0
0
1
1
REGULATION
OUT
TABLE 1. Frequency Modes
will ramp up in proportion to the rate that the
CS1
0
1
0
1
IN
and EN pins. During Soft Start, the
Frequency
300kHz
725kHz
OUT
9.5kHz
37kHz
to limit the gate
9
drive of the switched capacitor charge pump. This regulation
is done before the voltage is gained up by the charge pump,
giving rise to the term "pre-regulation". Pre-regulation helps
to reduce input current noise and large input current spikes
normally associated with switched capacitor charge pumps.
The LM2751 switched capacitor charge pump has gains of
2x and 1.5x. When the input voltage to the device is greater
than 3.58V (typ.), the LM2751 operates in a gain of 1.5x.
When the input voltage falls below 3.5V (typ.), the device
switches to a gain of 2x.
OUTPUT VOLTAGE RIPPLE
The primary contributor in keeping the output voltage ripple
of the LM2751 low is its switching topology. The output
capacitance, input voltage, switching frequency and output
current also play a significant part in determining the output
voltage ripple. Due to the complexity of the LM2751 opera-
tion, providing equations or models to approximate the mag-
nitude of the ripple cannot be easily accomplished. However,
the following general statements can be made.
The LM2751 has very low output ripple when compared to
typical boost regulators due to its double-pump topology,
where charge is continually supplied to the output during
both 2x and 1.5x modes. Combined with fixed frequency
operation modes, double-pumping allows for the use of a
very small, low value ceramic capacitor on the output node
while still achieving minimal output ripple. Increasing the
capacitance by adding a higher value capacitor or placing
multiple capacitors in parallel can further reduce the ripple
magnitude.
CAPACITOR SELECTION
The LM2751 requires 4 external capacitors for proper opera-
tion. Surface-mount multi-layer ceramic capacitors are rec-
ommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR, ≤15mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are generally not recommended
for use with the LM2751 due to their high ESR, as compared
to ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2751. These capacitors have tight capacitance tolerance
(as good as
85˚C), and typically have little voltage coefficient when com-
pared to other types of capacitors. However selecting a
capacitor with a voltage rating much higher than the voltage
it will be subjected to, will ensure that the capacitance will
stay closer to the capacitor’s nominal value. Capacitors with
Y5V or Z5U temperature characteristic are generally not
recommended for use with the LM2751. Capacitors with
these temperature characteristics typically have wide ca-
pacitance tolerance (+80%, −20%), vary significantly over
temperature (Y5V: +22%, −82% over −30˚C to +85˚C range;
Z5U: +22%, −56% over +10˚C to +85˚C range), and have
poor voltage coefficients. Under some conditions, a nominal
1µF Y5V or Z5U capacitor could have a capacitance of only
0.1µF. Such detrimental deviation is likely to cause Y5V and
Z5U capacitors to fail to meet the minimum capacitance
requirements of the LM2751.
±
15% over −55˚C to 125˚C; X5R:
±
10%), hold their value over temperature (X7R:
±
15% over −55˚C to
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