LM27964SQ-C/NOPB National Semiconductor, LM27964SQ-C/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM27964SQ-C/NOPB
• Description: IC LED DRIVR WHITE BCKLGT 24-LLP
• Manufacturer: National Semiconductor
• Type: Backlight, White LED (I²C Interface)r
• Series: -r

Specifications of LM27964SQ-C/NOPB

• Constant Current: Yes
• Topology: PWM, Switched Capacitor (Charge Pump)
• Number Of Outputs: 7
• Internal Driver: Yes
• Type - Primary: Backlight, General Purpose
• Type - Secondary: White LED
• Frequency: 500kHz ~ 900kHz
• Voltage - Supply: 2.7 V ~ 5.5 V
• Voltage - Output: 2 V ~ 4 V
• Mounting Type: Surface Mount
• Package / Case: 24-LLP
• Operating Temperature: -30°C ~ 85°C
• Current - Output / Channel: 30mA
• Internal Switch(s): Yes
• Efficiency: 87%
• Led Driver Application: Camera Phone LED Flash, LED Backlight
• No. Of Outputs: 7
• Output Current: 180mA
• Input Voltage: 2.7V To 5.5V
• Operating Temperature Range: -30°C To +85°C
• Rohs Compliant: Yes
• Constant Voltage: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM27964SQ-C; LM27964SQ-C; LM27964SQ-C-TR

forward voltage. Excessive power dissipation may also limit
output current capability of an application.
Total Output Current Capability
The maximum output current that can be drawn from the
LM27964 is 180mA. Each driver bank has a maximum allotted
current per Dxx sink that must not be exceeded.
The 180mA load can be distributed in many different config-
urations. Special care must be taken when running the
LM27964 at the maximum output current to ensure proper
functionality.
PARALLEL CONNECTED OUTPUTS
Outputs D1A-4A or D1B-D2B may be connected together to
drive one or two LEDs at higher currents. In such a configu-
ration, all four parallel current sinks (BankA) of equal value
can drive a single LED. The LED current programmed for
BankA should be chosen so that the current through each of
the outputs is programmed to 25% of the total desired LED
current. For example, if 60mA is the desired drive current for
a single LED, R
through each of the current sink inputs is 15mA. Similarly, if
two LEDs are to be driven by pairing up the D1A-4A inputs
(i.e D1A-2A, D3A-4A), R
current through each current sink input is 50% of the desired
LED current. The same RSETx selection guidelines apply to
BankB diodes.
Connecting the outputs in parallel does not affect internal op-
eration of the LM27964 and has no impact on the Electrical
Characteristics and limits previously presented. The available
diode output current, maximum diode voltage, and all other
specifications provided in the Electrical Characteristics table
apply to this parallel output configuration, just as they do to
the standard 4-LED application circuit.
Both BankA and BankB utilize LED forward voltage sensing
circuitry on each Dxx pin to optimize the charge-pump gain
for maximum efficiency. Due to the nature of the sensing cir-
cuitry, it is not recommended to leave any of the DxA or DxB
pins unused if either diode bank is going to be used during
normal operation. Leaving DxA and/or DxB pins unconnected
will force the charge-pump into 3/2× mode over the entire
V
by switching to 1× mode at higher input voltages.
Care must be taken when selecting the proper R
The current on any Dxx pin must not exceed the maximum
current rating for any given current sink pin.
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 LM27964 can
be predicted as follows:
IN
range negating any efficiency gain that could be achieve
DRIVER TYPE
(V
LEDB
DKEY
DxA
DxB
P
P
LEDTOTAL
IN
SETA
× N
= V
B
E = (P
IN
should be selected such that the current
× I
× (GAIN × I
IN
P
= (V
LEDB
SETA
). With a 1.5x/1x charge pump, the
IN
LEDTOTAL
= V
LEDA
) + (V
should be selected such that the
IN
MAXIMUM Dxx CURRENT
× N
× I
LED
LEDK
LEDTOTAL
÷ P
IN
30mA per DxA Pin
30mA per DxB Pin
A
) to the power drawn at
× I
IN
× N
)
LEDA
80mA
K
+ I
× I
) +
Q
LEDK
)
SETx
)
value.
11
It is also worth noting that efficiency as defined here is in part
dependent on LED voltage. Variation in LED voltage does not
affect power consumed by the circuit and typically does not
relate to the brightness of the LED. For an advanced analysis,
it is recommended that power consumed by the circuit (V
I
POWER DISSIPATION
The power dissipation (P
can be approximated with the equations below. P
power generated by the 1.5x/1x charge pump, P
power consumed by the LEDs, T
and θ
LLP-24 package. V
V
LEDs and I
The junction temperature rating takes precedence over the
ambient temperature rating. The LM27964 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 100°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 100°C.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM27964
when the junction temperature exceeds 170°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 165°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 LM27964 requires 4 external capacitors for proper oper-
ation (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 LM27964 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
LM27964. 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 LM27964. 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
IN
LED
) be evaluated rather than power efficiency.
P
DISS
is the nominal LED forward voltage, N is the number of
JA
= (GAIN × V
1
is the junction-to-ambient thermal resistance for the
(V
= C
LED
LEDB
P
2
DISS
is the programmed LED current.
= 1µF, C
× N
= P
T
IN
B
IN
J
× I
= T
× I
IN
is the input voltage to the LM27964,
LEDB
- P
IN
LEDA + LEDB + LEDK
DISS
A
I
+ (P
= C
LEDA
LEDA
) - (V
) and junction temperature (T
OUT
DISS
) -
A
- P
is the ambient temperature,
LEDK
= 2.2µF). Surface-mount
LEDB
x θ
× N
JA
- P
)
) - (V
K
LEDK
× I
LEDK
LEDA
www.national.com
LED
)
× N
IN
is the
is the
A
×
IN
J
x
)