lm2756tmx National Semiconductor Corporation, lm2756tmx Datasheet - Page 13

• Manufacturer Part Number: lm2756tmx
• Description: Multi-display Inductorless Led Driver With 32 Exponential Dimming Steps In ?smd
• Manufacturer: National Semiconductor Corporation

current (total LED current). The efficiency of the LM2756 can
be predicted as follow:
The LED voltage is the main contributor to the charge-pump
gain selection process. Use of low forward-voltage LEDs
(3.0V- to 3.5V) will allow the LM2756 to stay in the gain of 1×
for a higher percentage of the lithium-ion battery voltage
range when compared to the use of higher forward voltage
LEDs (3.5V to 4.0V). See the LED Forward Voltage Monitor-
ing section of this datasheet for a more detailed description
of the gain selection and transition process.
For an advanced analysis, it is recommended that power con-
sumed by the circuit (V
rather than power efficiency.
POWER DISSIPATION
The power dissipation (P
can be approximated with the equations below. P
power generated by the 3/2× - 1× charge pump, P
power consumed by the LEDs, T
and θ
µSMD 20-bump package. V
LM2756, V
number of LEDs and I
P
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2756 may be operated
outside the ambient temperature rating, so long as the junc-
tion temperature of the device does not exceed the maximum
operating rating of 105°C. The maximum ambient tempera-
ture rating must be derated in applications where high power
DISS
= (GAIN × V
JA
I
is the junction-to-ambient thermal resistance for the
LEDA
LED
(V
) - (V
P
P
P
LEDB
LEDTOTAL
IN
DISS
is the nominal LED forward voltage, N is the
= V
IN
LEDB
× N
= P
T
E = (P
IN
× I
J
= T
B
LED
× (GAIN × I
IN
GroupA + GroupB + GroupC
× N
IN
P
= (V
× I
- P
IN
DISS
A
x I
LEDTOTAL
is the programmed LED current.
B
LEDB
+ (P
= V
LEDA
LEDA
IN
× I
) and junction temperature (T
) for a given load be evaluated
IN
IN
LEDB
) + (V
DISS
A
is the input voltage to the
- P
× N
× I
is the ambient temperature,
LEDTOTAL
) - (V
÷ P
LEDB
IN
x θ
A
LEDC
× I
IN
JA
LEDC
)
- P
LEDA
)
× I
+ I
LEDC
) - (V
LEDC
) +
× I
Q
)
LEDC
)
LEDA
LED
)
IN
× N
is the
is the
A
J
×
)
13
dissipation and/or poor thermal resistance causes the junc-
tion temperature to exceed 105°C.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM2756
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 pow-
er dissipation. The device will recover and operate normally
when the junction temperature falls below 155°C (typ.). It is
important that the board layout provide good thermal conduc-
tion to keep the junction temperature within the specified
operating ratings.
CAPACITOR SELECTION
The LM2756 requires 4 external capacitors for proper opera-
tion (C
ceramic capacitors are recommended. These capacitors are
small, inexpensive and have very low equivalent series re-
sistance (ESR <20mΩ typ.). Tantalum capacitors, OS-CON
capacitors, and aluminum electrolytic capacitors are not rec-
ommended for use with the LM2756 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
LM2756. These capacitors have tight capacitance tolerance
(as good as ±10%) and hold their value over temperature
(X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to
85°C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2756. Ca-
pacitors with these temperature characteristics typically have
wide capacitance tolerance (+80%, -20%) and vary signifi-
cantly 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 de-
viation is likely to cause Y5V and Z5U capacitors to fail to
meet the minimum capacitance requirements of the LM2756.
The recommended voltage rating for the capacitors is
10V to account for DC bias capacitance losses.
1
= C
2
= C
IN
= C
OUT
= 1µF). Surface-mount multi-layer
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