Datasheet
P
COND
= (I
OUT
2
x D) x
1
3
1 +
x
'i
L
I
OUT
2
x R
DS (ON)
D =
V
OUT
+ V
D
+ V
DCR
V
IN
+ V
D
- V
SW
D =
V
OUT
+ V
D
V
IN
+ V
D
- V
SW
K =
P
OUT
P
OUT
+ P
LOSS
K =
P
OUT
P
IN
LM2833
www.ti.com
SNVS505E –MAY 2008–REVISED APRIL 2013
EFFICIENCY ESTIMATION
The complete LM2833 DC/DC converter efficiency can be calculated in the following manner:
(17)
Or
(18)
Calculations for determining the most significant power losses are shown below. Other losses totaling less than
2% are not discussed.
The main power loss (P
LOSS
) in the converter includes two basic types of losses: switching loss and conduction
loss. In addition, there is loss associated with the power required for the internal circuitry of IC. Conduction
losses usually dominate at higher output loads, whereas switching losses dominate at lower output loads. The
first step in determining the losses is to calculate the duty cycle (D):
(19)
V
SW
is the voltage drop across the internal power switch when it is on, and is equal to:
V
SW
= I
OUT
x R
DS(ON)
(20)
V
D
is the forward voltage drop across the catch diode. It can be obtained from the diode manufactures Electrical
Characteristics section. If the DC voltage drop across the inductor (V
DCR
) is accounted for, the equation
becomes:
(21)
The conduction losses in the catch diode are calculated as follows:
P
DIODE
= V
D
x I
OUT
x (1-D) (22)
Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky
diode with a low forward voltage drop.
Another significant external power loss is the conduction loss in the output inductor. The equation can be
simplified to:
P
IND
= I
OUT
2
x R
DCR
(23)
The LM2833 conduction loss is mainly associated with the internal power switch:
(24)
If the inductor ripple current is fairly small, the conduction losses can be simplified to:
P
COND
= I
OUT
2
x R
DS(ON)
x D (25)
Switching losses are also associated with the internal power switch. They occur during the switch on and off
transition periods, 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.
Switching Power Loss is calculated as follows:
P
SWR
= 0.5 x (V
IN
x I
OUT
x f
SW
x T
RISE
) (26)
P
SWF
= 0.5 x (V
IN
x I
OUT
x f
SW
x T
FALL
) (27)
P
SW
= P
SWR
+ P
SWF
(28)
The power loss required for operation of the internal circuitry is given by:
P
Q
= I
Q
x V
IN
(29)
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