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參數(shù)資料
| 型號： | LM27952SDX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 顯示驅(qū)動器 |
| 英文描述： | White LED Adaptive 1.5X/1X Switched Capacitor Current Driver |
| 中文描述： | LED DISPLAY DRIVER, PDSO14 |
| 封裝： | 4 X 3 MM, 0.80 MM HEIGHT, LLP-14 |
| 文件頁數(shù)： | 6/8頁 |
| 文件大?。?/td> | 521K |
| 代理商： | LM27952SDX |
Application Information
(Continued)
The equation below can be used to estimate the total output
current capability of the LM27952:
I
LED_MAX
= ((1.5 x V
IN
) - V
LED
) / ((N x R
OUT
) + k
HR
) (eq. 1)
I
LED_MAX
= ((1.5 x V
IN
) - V
LED
) / ((N x 3.3
) + 12mV/mA)
R
– Output resistance. This parameter models the inter-
nal losses of the charge pump that result in voltage droop at
the pump output V
. Since the magnitude of the voltage
droop is proportional to the total output current of the charge
pump, the loss parameter is modeled as a resistance. The
output resistance of the LM27952 is typically 3.3
(V
IN
=
3.0V, T
A
= 25C). In equation form:
V
VOUT
= 1.5 x V
IN
– N x I
LED
x R
OUT
k
– Headroom constant. This parameter models the mini-
mum voltage required across the current sinks for proper
regulation. This minimum voltage is proportional to the pro-
grammed LED current, so the constant has units of mV/mA.
The typical k
HR
of the LM27952 is 12mV/mA. In equation
form:
(V
VOUT
– V
LED
)
>
k
HR
x I
LED
The "I
LED-MAX
" equation (eq. 1) is obtained from combining
the R
equation (eq. 3) and
solving for I
. Maximum LED current is highly dependent
on minimum input voltage and LED forward voltage. Output
current capability can be increased by raising the minimum
input voltage of the application, or by selecting LEDs with a
lower forward voltage. Excessive power dissipation may also
limit output current capability of an application.
(eq. 2)
(eq. 3)
CAPACITOR SELECTION
The LM27952 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
<
20m
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are not recommended for use
with the LM27952 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
LM27952. These capacitors have tight capacitance toler-
ance (as good as
±
10%) and hold their value over tempera-
ture (X7R:
±
15% over -55C to 125C; X5R:
±
15% over
-55C to 85C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM27952. Ca-
pacitors with these temperature characteristics typically
have wide capacitance tolerance (+80%, -20%) and vary
significantly over temperature (Y5V: +22%, -82% over -30C
to +85C range; Z5U: +22%, -56% over +10C to +85C
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 LM27952.
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.
PARALLEL DX OUTPUTS FOR INCREASED CURRENT
DRIVE
Outputs D
may be connected together to drive a one or
two LEDs at higher currents. In such a configuration, all four
parallel current sinks of equal value drive the single LED.
The LED current programmed 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 the single LED, R
should be
selected such that the current through each of the current
sink inputs is 15mA. Similarly, if two LEDs are to be driven by
pairing up the D
1-4
inputs (i.e D
1-2
, D
3-4
), R
SET
should be
selected such that the current through each current sink
input is 50% of the desired LED current.
Connecting the outputs in parallel does not affect internal
operation of the LM27952 and has no impact on the Electri-
cal Characteristics and limits previously presented. The
available diode output current, maximum diode voltage, and
all other specifications provided in the Electrical Character-
istics table apply to this parallel output configuration, just as
they do to the standard 4-LED application circuit.
POWER EFFICIENCY
Efficiency of LED drivers is commonly taken to be the ratio of
power consumed by the LEDs (PLED) to the power drawn at
the input of the part (PIN). With a 1.5x/1x charge pump, the
input current is equal to the charge pump gain times the
output current (total LED current). For a simple approxima-
tion, the current consumed by internal circuitry can be ne-
glected and the efficiency of the LM27952 can be predicted
as follows:
P
LED
= N x V
LED
x I
LED
P
IN
= V
IN
x I
IN
P
IN
= V
IN
x (Gain x N x I
LED
+ I
Q
)
E = (P
LED
÷ P
IN
)
Neglecting I
will result in a slightly higher efficiency predic-
tion, but this impact will be no more than a few percentage
points when several LEDs are driven at full power. It is also
worth noting that efficiency as defined here is in part depen-
dent 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 analy-
sis, it is recommended that power consumed by the circuit
(V
IN
x I
IN
) be evaluated rather than power efficiency.
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM27952
when the junction temperature exceeds 150C (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 140C (typ.).
It is important that the board layout provide good thermal
conduction to keep the junction temperature within the speci-
fied operating ratings.
POWER DISSIPATION
The power dissipation (P
) and junction tempera-
ture (T
) can be approximated with the equations below. P
IN
is the power generated by the 1.5x/1x charge pump, P
is
the power consumed by the LEDs, T
is the ambient tem-
perature, and
θ
is the junction-to-ambient thermal resis-
tance for the LLP-14 package. V
IN
is the input voltage to the
LM27952, V
LED
is the nominal LED forward voltage, and
I
LED
is the programmed LED current.
P
DISSIPATION
= P
IN
- P
LED
= [Gain x V
IN
x (4 x I
LED
)] (V
LED
x 4 x I
LED
)
T
J
= T
A
+ (P
DISSIPATION
x
θ
JA
)
L
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