LM3410_08 Datasheet, PDF (15/32 Pages) National Semiconductor (TI) – PowerWise® 525kHz/1.6MHz, Constant Current Boost and SEPIC LED Driver with Internal Compensation

P_IND = I_IN2R_DCR
or
The LM3410 conduction loss is mainly associated with the
internal power switch:
P_COND-NFET = I^2SW-rms x R_DSON x D
30038542
FIGURE 11. LM3410 Switch Current
(small ripple approximation)
P_COND-NFET = I_IN2 x R_DSON x D
or
The value for R_DSON should be equal to the resistance at the
junction temperature you wish to analyze. As an example, at
125°C and R_DSON = 250 mΩ (See typical graphs for value).
Switching losses are also associated with the internal power
switch. They occur during the switch on and off transition pe-
riods, where voltages and currents overlap resulting in power
loss.
The simplest means to determine this loss is to empirically
measuring the rise and fall times (10% to 90%) of the switch
at the switch node:
P_SWR = 1/2(V_OUT x I_IN x f_SW x t_RISE)
P_SWF = 1/2(V_OUT x I_IN x f_SW x t_FALL)
P_SW = P_SWR + P_SWF
Typical Switch-Node Rise and Fall Times
V_IN
V_OUT
t_RISE
t_FALL
3V
5V
6nS
4nS
5V
12V
6nS
5nS
3V
12V
8nS
7nS
5V
18V
10nS
8nS
Quiescent Power Losses
I_Q is the quiescent operating current, and is typically around
1.5 mA.
R_SET Power Loss
P_Q = I_Q x V_IN
Example Efficiency Calculation:
Operating Conditions:
5 x 3.3V LEDs + 190mV_REF ≊ 16.7V
TABLE 1. Operating Conditions
V_IN
V_OUT
I_LED
V_D
f_SW
I_Q
t_RISE
t_FALL
R_DSON
L_DCR
D
I_IN
3.3V
16.7V
50mA
0.45V
1.60MHz
3mA
10nS
10nS
225mΩ
75mΩ
0.82
0.31A
ΣP_COND + P_SW + P_DIODE + P_IND + P_Q = P_LOSS
Quiescent Power Loss:
P_Q = I_Q x V_IN = 10 mW
Switching Power Loss:
P_SWR = 1/2(V_OUT x I_IN x f_SW x t_RISE) ≊ 40 mW
P_SWF = 1/2(V_OUT x I_IN x f_SW x t_FALL) ≊ 40 mW
P_SW = P_SWR + P_SWF = 80 mW
Internal NFET Power Loss:
R_DSON = 225 mΩ
P_CONDUCTION = I_IN2 x D x R_DSON = 17 mW
I_IN = 310 mA
Diode Loss:
V_D = 0.45V
P_DIODE = V_D x I_LED = 23 mW
Inductor Power Loss:
R_DCR = 75 mΩ
P_IND = I_IN2 x R_DCR = 7 mW
15
www.national.com