lm2755tmx National Semiconductor Corporation, lm2755tmx Datasheet - Page 11

• Manufacturer Part Number: lm2755tmx
• Description: Charge Pump Led Controller With I2c Compatible Interface In Micro Smd
• Manufacturer: National Semiconductor Corporation

Total Output Current Capability
The maximum output current that can be drawn from the
LM2755 is 90mA. Each driver Group has a maximum allotted
current per Dx sink that must not be exceeded.
The 90mA load can be distributed in many different configu-
rations. Special care must be taken when running the LM2755
at the maximum output current to ensure proper functionality.
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
input current is equal to the charge pump gain times the output
current (total LED current). The efficiency of the LM2755 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 LM2755 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 18-bump package. V
LM2755, V
number of LEDs and I
P
DISS
= (GAIN × V
JA
DRIVER TYPE
is the junction-to-ambient thermal resistance for the
LED
(V
P
P
P
Dx
LEDB
LEDTOTAL
IN
DISS
is the nominal LED forward voltage, N is the
= V
IN
× N
= P
I
E = (P
LED2
IN
× I
B
LED
IN
× (GAIN × I
IN
D1 + D2+ D3
IN
P
= (V
× I
). With a 3/2× - 1× charge pump, the
) - (V
IN
- P
DISS
x I
LEDTOTAL
is the programmed LED current.
LEDB
= V
LEDA
LED1
IN
LED3
) and junction temperature (T
) for a given load be evaluated
IN
IN
) + (V
A
MAXIMUM Dx CURRENT
is the input voltage to the
× N
- P
) - (V
× I
is the ambient temperature,
LED
× I
LEDTOTAL
÷ P
IN
LED2
A
LEDC
LED3
30mA per Dx Pin
) to the power drawn at
LED1
× I
IN
)
- P
)
LEDA
× I
× I
+ I
LED3
LEDC
) +
LED1
Q
)
)
) - (V
LED
IN
LED2
is the
is the
J
×
)
11
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2755 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
dissipation and/or poor thermal resistance causes the junc-
tion temperature to exceed 105°C.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM2755
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 LM2755 requires 4 external capacitors for proper opera-
tion (C
multi-layer ceramic capacitors are recommended. These ca-
pacitors are small, inexpensive and have very low equivalent
series resistance (ESR <20mΩ typ.). Tantalum capacitors,
OS-CON capacitors, and aluminum electrolytic capacitors are
not recommended for use with the LM2755 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
LM2755. 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 LM2755. 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 LM2755.
The recommended voltage rating for the capacitors is
10V to account for DC bias capacitance losses.
IN
= C
OUT
= 1µF, C
T
J
= T
A
1
+ (P
= C
DISS
2
= 0.47µF). Surface-mount
x θ
JA
)
www.national.com