LM26420, LM26420-Q0, LM26420-Q1
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SNVS579J –FEBRUARY 2009–REVISED SEPTEMBER 2015
POUT
K =
PIN
(17)
(18)
Or
POUT
POUT + PLOSS
K =
Calculations for determining the most significant power losses are shown below. Other losses totaling less than
2% are not discussed.
Power loss (PLOSS) is the sum of two basic types of losses in the converter: switching and conduction.
Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and
dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D):
VOUT + VSW_BOT
D =
VIN + VSW_BOT ± VSW_TOP
(19)
VSW_TOP is the voltage drop across the internal PFET when it is on, and is equal to:
VSW_TOP = IOUT × RDSON_TOP
(20)
VSW_BOT is the voltage drop across the internal NFET when it is on, and is equal to:
VSW_BOT = IOUT × RDSON_BOT
(21)
(22)
If the voltage drop across the inductor (VDCR) is accounted for, the equation becomes:
VOUT + VSW_BOT + VDCR
D =
VIN + VSW_BOT + VDCR ± VSW_TOP
Another significant external power loss is the conduction loss in the output inductor. The equation can be
simplified to:
PIND = IOUT2 x RDCR
(23)
The LM26420 conduction loss is mainly associated with the two internal FETs:
2
'iL
1
3
PCOND_TOPꢀ= (IOUT2 x D)
x
RDSON_TOP
1 +
IOUT
2
'iL
IOUT
1
3
PCOND_BOTꢀ= (IOUT2 x (1-D))
x
RDSON_BOT
1 +
(24)
If the inductor ripple current is fairly small, the conduction losses can be simplified to:
PCOND_TOP = (IOUT2 × RDSON_TOP × D)
(25)
(26)
(27)
PCOND_BOT = (IOUT2 × RDSON_BOT × (1-D))
PCOND = PCOND_TOP + PCOND_BOT
Switching losses are also associated with the internal FETs. 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:
PSWR = 1/2(VIN × IOUT × FSW × TRISE
)
(28)
(29)
(30)
PSWF = 1/2(VIN × IOUT × FSW × TFALL
PSW = PSWR + PSWF
)
Another loss is the power required for operation of the internal circuitry:
PQ = IQ × VIN
(31)
23
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