lp3205 Lowpower Semiconductor inc, lp3205 Datasheet - Page 8

• Manufacturer Part Number: lp3205
• Description: 1.5mhz,800ma,high Efficiency Synchronous Pwm Step-down Dc/dc Convert
• Manufacturer: Lowpower Semiconductor inc

Efficiency= 100%- (L1+L2+L3…)
Where L1、L2, etc. are the individual losses as a percentage of
Input power .although all dissipative elements in the for most
of losses: VIN quiescent current and 1
efficiency loss at medium to high load currents. In a typical
efficiency plot, the efficiency curve at very low load currents
can be misleading since the actual power lost is of no
consequence.
1.The VIN quiescent current is due to two components:
the DC Bias current as given in the electrical
characteristics and the Internal main switch and
synchronous switch gate charge currents. the gate
charge current results from switching the gate
capacitance
switches .Each time the gate charge current. results
from switching the gate capacitance of the internal
power MOSFET switches. Each time the gate is
switches from high to low to high again, a packet of
charge △Q moves from V
The resulting △Q/△t is the current out of V
larger than the DC bias current. In continuous mode.
Where Q
bottom switches. Both the DC bias and gate charge losses are
proportional to V
pronounced at higher supply voltages.
LP3205 – Ver. 1.0 Datasheet
T
and Q
L
of
B
GATCHG
are the gate charges of the internal top and
IN
the
and thus their effects will be more
=f(Q
internal
IN
T
+Q
Feb.-2007
to ground.
B
)
2
Preliminary Datasheet
R loss dominates the
power
IN
that is typically
MOSFET
2. 1
switches, R
the average output current flowing through inductor L is
“chopped” between the main switch and the synchronous
switch. Thus, the series resistance looking into the LX pin is a
function of both top and bottom MOSFER R
cycle (DC) as follows:
The R
obtained from the typical performance characteristics curves.
thus, to obtain 1
the square of the average output current.
Other losses including C
losses and inductor core losses generally account for
less than 2% of the total loss.
2
Rlosses tae calculated from the resistances of the internal
DS(ON)
SW
for both the top and bottom MOSFETS can be
and external inductor R
2
R losses, simply add R
IN
and C
Page 8 of 10
OUT
L
LP3205
SW
. in continuous mode
to R
DS(ON)
ESR dissipative
L
and multiply
and the duty