LMR10510_14 Datasheet, PDF (13/26 Pages) Texas Instruments – SIMPLE SWITCHER® 5.5Vin, 1A Step-Down Voltage Regulator in SOT-23 and WSON

LMR10510
www.ti.com
SNVS727A – OCTOBER 2011 – REVISED OCTOBER 2011
V_OUT + V_D + V_DCR
D=
V_IN + V_D + V_DCR - V_SW
The conduction losses in the free-wheeling Schottky diode are calculated as follows:
P_DIODE = V_D x I_OUT x (1-D)
Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky
diode that has 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_OUT2 x R_DCR
The LMR10510 conduction loss is mainly associated with the internal PFET:
P_COND =
(I_OUT2
x
D)
1
+
1
3
x
'i_L
I_OUT
2
R_DSON
If the inductor ripple current is fairly small, the conduction losses can be simplified to:
P_COND = I_OUT2 x R_DSON x D
Switching losses are also associated with the internal PFET. 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 = 1/2(V_IN x I_OUT x F_SW x T_RISE)
P_SWF = 1/2(V_IN x I_OUT x F_SW x T_FALL)
P_SW = P_SWR + P_SWF
Another loss is the power required for operation of the internal circuitry:
P_Q = I_Q x V_IN
I_Q is the quiescent operating current, and is typically around 3.3mA for the 1.6MHz frequency option.
Typical Application power losses are:
Table 1. Power Loss Tabulation
V_IN
V_OUT
I_OUT
V_D
F_SW
I_Q
T_RISE
T_FALL
R_DS(ON)
IND_DCR
D
5.0V
3.3V
1.0A
0.45V
1.6MHz
3.3mA
4nS
4nS
150mΩ
70mΩ
0.667
η
88%
ΣP_COND + P_SW + P_DIODE + P_IND + P_Q = P_LOSS
ΣP_COND + P_SWF + P_SWR + P_Q = P_INTERNAL
P_INTERNAL = 149mW
P_OUT
P_DIODE
P_Q
P_SWR
P_SWF
P_COND
P_IND
P_LOSS
P_INTERNAL
3.3W
150mW
17mW
16mW
16mW
100mW
70mW
369mW
149mW
Copyright © 2011, Texas Instruments Incorporated
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