LP3205 Datasheet, PDF (8/10 Pages) Lowpower Semiconductor inc – 1.5MHZ,800mA,High Efficiency Synchronous PWM Step-Down DC/DC Convert

Preliminary Datasheet
LP3205
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 1²R loss dominates the
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 of the internal power MOSFET
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_IN to ground.
The resulting △Q/△t is the current out of V_IN that is typically
larger than the DC bias current. In continuous mode.
L_GATCHG=f(Q_T+Q_B)
Where Q_T and Q_B are the gate charges of the internal top and
bottom switches. Both the DC bias and gate charge losses are
proportional to V_IN and thus their effects will be more
pronounced at higher supply voltages.
2. 1²Rlosses tae calculated from the resistances of the internal
switches, R_SW and external inductor R_L. in continuous mode
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_DS(ON) and the duty
cycle (DC) as follows:
The R_DS(ON) for both the top and bottom MOSFETS can be
obtained from the typical performance characteristics curves.
thus, to obtain 1²R losses, simply add R_SW to R_L and multiply
the square of the average output current.
Other losses including C_IN and C_OUT ESR dissipative
losses and inductor core losses generally account for
less than 2% of the total loss.
LP3205 – Ver. 1.0 Datasheet Feb.-2007
Page 8 of 10