Texas Instruments LM3402HV Evaluation Board LM3402HVEVAL/NOPB LM3402HVEVAL/NOPB Datenbogen
Produktcode
LM3402HVEVAL/NOPB
R
ON
=
V
O
1.34 x 10
-10
x f
SW
SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013
Gate charging and VCC loss, P
G
, in the gate drive and linear regulator:
P
G
= (I
IN-OP
+ f
SW
x Q
G
) x V
IN
P
G
= (600 x 10
-6
+ 468000 x 3 x 10
-9
) x 24 = 48 mW
(41)
Switching loss, P
S
, in the internal MOSFET:
P
S
= 0.5 x V
IN
x I
F
x (t
R
+ t
F
) x f
SW
P
S
= 0.5 x 24 x 0.35 x (40 x 10
-9
) x 468000 = 78 mW
(42)
AC rms current loss, P
CIN
, in the input capacitor:
P
CIN
= I
IN(rms)
2
x ESR = (0.126)
2
x 0.006 = 0.1 mW (negligible)
(43)
DCR loss, P
L
, in the inductor
P
L
= I
F
2
x DCR = 0.35
2
x 0.096 = 11.8 mW
(44)
Recirculating diode loss, P
D
= 119 mW
Current Sense Resistor Loss, P
SNS
= 92 mW
Electrical efficiency,
η
= P
O
/ (P
O
+ Sum of all loss terms) = 1.295 / (1.295 + 0.377) = 77%
DIE TEMPERATURE
T
LM3402
= (P
C
+ P
G
+ P
S
) x
θ
JA
T
LM3402
= (0.028 + 0.05 + 0.078) x 200 = 31°C
(45)
Design Example 2: LM3402HV
The second example application is an RGB backlight for a flat screen monitor. A separate boost regulator
provides a 60V ±5% DC input rail that feeds three LM3402HV current regulators to drive one series array each of
red, green, and blue 1W LEDs. The target for average LED current is 350 mA ±5% in each string. The monitor
will adjust the color temperature dynamically, requiring fast PWM dimming of each string with external, parallel
MOSFETs. 1W green and blue InGaN LEDs have a typical forward voltage of 3.5V, however red LEDs use
AlInGaP technology with a typical forward voltage of 2.9V. In order to match color properly the design requires
14 green LEDs, twice as many as needed for the red and blue LEDs. This example will follow the design for the
green LED array, providing the necessary information to repeat the exercise for the blue and red LED arrays.
The circuit schematic for Design Example 2 is the same as the Typical Application on the front page. The bill of
materials (green array only) can be found in Table 2 at the end of this datasheet.
provides a 60V ±5% DC input rail that feeds three LM3402HV current regulators to drive one series array each of
red, green, and blue 1W LEDs. The target for average LED current is 350 mA ±5% in each string. The monitor
will adjust the color temperature dynamically, requiring fast PWM dimming of each string with external, parallel
MOSFETs. 1W green and blue InGaN LEDs have a typical forward voltage of 3.5V, however red LEDs use
AlInGaP technology with a typical forward voltage of 2.9V. In order to match color properly the design requires
14 green LEDs, twice as many as needed for the red and blue LEDs. This example will follow the design for the
green LED array, providing the necessary information to repeat the exercise for the blue and red LED arrays.
The circuit schematic for Design Example 2 is the same as the Typical Application on the front page. The bill of
materials (green array only) can be found in Table 2 at the end of this datasheet.
OUTPUT VOLTAGE
Green Array: V
O(G)
= 14 x 3.5 + 0.2 = 49.2V
(46)
Blue Array: V
O(B)
= 7 x 3.5 + 0.2 = 24.7V
(47)
Red Array: V
O(R)
= 7 x 2.9 + 0.2 = 20.5V
(48)
R
ON
and t
ON
A compromise in switching frequency is needed in this application to balance the requirements of magnetics size
and efficiency. The high duty cycle translates into large conduction losses and high temperature rise in the IC.
For best response to a PWM dimming signal this circuit will not use an output capacitor; hence a moderate
switching frequency of 300 kHz will keep the inductance from becoming so large that a custom-wound inductor is
needed. This design will use only surface mount components, and the selection of off-the-shelf SMT inductors for
switching regulators is poor at 1000 µH and above. R
and efficiency. The high duty cycle translates into large conduction losses and high temperature rise in the IC.
For best response to a PWM dimming signal this circuit will not use an output capacitor; hence a moderate
switching frequency of 300 kHz will keep the inductance from becoming so large that a custom-wound inductor is
needed. This design will use only surface mount components, and the selection of off-the-shelf SMT inductors for
switching regulators is poor at 1000 µH and above. R
ON
is selected from the equation for switching frequency as
follows:
(49)
R
ON
= 49.2 / (1.34 x 10
-10
x 3 x 10
5
) = 1224 k
Ω
(50)
The closest 1% tolerance resistor is 1.21 M
Ω
. The switching frequency and on-time of the circuit can then be
found using the equations relating R
ON
and t
ON
to f
SW
:
f
SW
= 49.2 / (1210000 x 1.34 x 10
-10
) = 303 kHz
(51)
t
ON
= (1.34 x 10
-10
x 1210000) / 60 = 2.7 µs
(52)
USING AN OUTPUT CAPACITOR
This application is dominated by the need for fast PWM dimming, requiring a circuit without any output
capacitance.
capacitance.
26
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