lm27953tlx National Semiconductor Corporation, lm27953tlx Datasheet - Page 10

• Manufacturer Part Number: lm27953tlx
• Description: White Led Driver With Four Led Current Sinks And 3/2x Switched Capacitor Boost
• Manufacturer: National Semiconductor Corporation

www.national.com
Circuit Description
outputs rise at the rate of the reference voltage ramp. Due to
the soft-start circuitry, turn-on time of the LM27953 is ap-
proximately 350µs (typ.).
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM27953
when the junction temperature exceeds 160˚C (typ.). This
feature protects the device from being damaged by high die
temperatures that might otherwise result from excessive
power dissipation. The device will recover and operate nor-
mally when the junction temperature falls below 120˚C (typ.).
It is important that the board layout provides good thermal
conduction. This will help to keep the junction temperature
within specified operating ratings.
Applications Information
POWER EFFICIENCY
Efficiency of LED drivers is commonly taken to be the ratio of
power consumed by the LEDs (P
the input of the part (P
current is approximately 1.5x the output current (total LED
current). For a simple approximation, the current consumed
by internal circuitry can be neglected and the efficiency of
the LM27953 can be predicted as follows:
Neglecting I
tion, but this impact will be no more than a few percentage
points when several LEDs are driven at full power.
ADJUSTING LED BRIGHTNESS (PWM control)
Perceived LED brightness can be adjusted using a PWM
control signal to turn the LM27953 current sources ON and
OFF at a rate faster than perceptible by the eye. When this
is done, the total brightness perceived is proportional to the
duty cycle (D) of the PWM signal (D = the percentage of time
that the LED is on in every PWM cycle). A simple example: if
the LEDs are driven at 15mA each with a PWM signal that
has a 50% duty cycle, perceived LED brightness will be
about half as bright as compared to when the LEDs are
driven continuously with 15mA. A PWM signal thus provides
brightness (dimming) control for the solution.
The minimum recommended PWM frequency is 100Hz. Fre-
quencies below this may be visibly noticeable as flicker or
blinking. The maximum recommended PWM frequency is
1kHz. Frequencies above this may cause interference with
internal current driver circuitry.
In cases where a PWM signal must be connected to the EN
pin, measures can be taken to reduce the magnitude of the
Q
will result in a slightly higher efficiency predic-
IN
). With a 1.5x charge pump, the input
LED
(Continued)
) to the power drawn at
10
charge-pump turn-on voltage spikes. More input capaci-
tance, series resistors and/or ferrite beads may provide ben-
efits.
If the current and voltage spikes can be tolerated, connect-
ing the PWM signal to the EN pin does provide a benefit:
lower supply current when the PWM signal is active. When
the PWM signal is low, the LM27953 will be shutdown and
input current will only be a few micro-amps. This results in a
lower time-averaged input current.
CAPACITOR SELECTION
The LM27953 requires 4 external capacitors for proper op-
eration. Surface-mount multi-layer ceramic capacitors are
recommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are not recommended for use
with the LM27953 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
LM27953. These capacitors have tight capacitance toler-
ance (as good as
ture (X7R:
-55˚C to 85˚C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM27953. Ca-
pacitors with these temperature characteristics typically
have wide capacitance tolerance (+80%, -20%) and vary
significantly over temperature (Y5V: +22%, -82% over -30˚C
to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C
range). 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 capaci-
tors to fail to meet the minimum capacitance requirements of
the LM27953.
The voltage rating of the output capacitor should be 10V or
more. All other capacitors should have a voltage rating at or
above the maximum input voltage of the application.
CIRCUIT BOARD LAYOUT
For optimal, low-noise performance, all capacitors (C
C
A solid ground plane should be used for IC and component
GND connections. Refer to the LM27953 Evaluation Board
for an example layout.
MICRO SMD MOUNTING
The LM27953 is an 18-bump micro SMD with a bump size of
approximately 300 micron diameter. The micro SMD pack-
age requires specific mounting techniques detailed in Na-
tional Semiconductor Application Note 1112 (AN-1112).
POUT
, C1, C2) should be placed very close to the LM27953.
±
15% over -55˚C to 125˚C; X5R:
±
10%) and hold their value over tempera-
±
15% over
<
20mW
IN
,