lm27965sqx-m National Semiconductor Corporation, lm27965sqx-m Datasheet - Page 12

• Manufacturer Part Number: lm27965sqx-m
• Description: Dual Display White Led Driver With I2c Compatible Brightness Control
• Manufacturer: National Semiconductor Corporation

www.national.com
V
LEDs and I
The junction temperature rating takes precedence over the
ambient temperature rating. The LM27965 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 LM27965
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 LM27965 requires 4 external capacitors for proper oper-
ation (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 LM27965 due to their high ESR,
as compared to ceramic capacitors.
P
LED
DISS
is the nominal LED forward voltage, N is the number of
= (GAIN × V
I
1
LEDA
= C
LED
) - (V
2
P
= C
DISS
is the programmed LED current.
IN
IN
LEDB
= P
T
= C
× I
J
= T
IN
BANKA + BANKB + BANKC
× N
OUT
- P
A
B
+ (P
= 1µF). Surface-mount multi-layer
LEDA
× I
LEDB
DISS
- P
) - (V
LEDB
x θ
JA
LEDC
- P
)
LEDC
) - (V
× I
LEDC
LEDA
)
× N
A
×
12
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM27965. 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 LM27965. 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
LM27965.
The minimum voltage rating acceptable for all capacitors
is 6.3V. The recommended voltage rating for the capaci-
tors is 10V to account for DC bias capacitance losses.
PCB LAYOUT CONSIDERATIONS
The LLP is a leadframe based Chip Scale Package (CSP)
with very good thermal properties. This package has an ex-
posed DAP (die attach pad) at the center of the package
measuring 2.6mm x 2.5mm. The main advantage of this ex-
posed DAP is to offer lower thermal resistance when it is
soldered to the thermal land on the PCB. For PCB layout,
National highly recommends a 1:1 ratio between the package
and the PCB thermal land. To further enhance thermal con-
ductivity, the PCB thermal land may include vias to a ground
plane. For more detailed instructions on mounting LLP pack-
ages, please refer to National Semiconductor Application
Note AN-1187.