参数资料
| 型号： | LTC3819EG#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 22/32页 |
| 文件大小： | 0K |
| 描述： | IC CNTRLR STEP DOWN 36-SSOP |
| 标准包装： | 2,000 |
| 应用： | 控制器，Sun 服务器 |
| 输入电压： | 4 V ~ 36 V |
| 输出数： | 1 |
| 输出电压： | 1.03 V ~ 1.41 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 36-SSOP（0.209"，5.30mm 宽） |
| 供应商设备封装： | 36-SSOP |
| 包装： | 带卷 (TR) |
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LTC3819
APPLICATIO S I FOR ATIO
Transition Loss = ( 1 . 7 ) V IN ?
? C RSS f
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
%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 circuit produce
losses, four main sources usually account for most of the
losses in LTC3819 circuits: 1) I 2 R losses, 2) Topside
MOSFET transition losses, 3) INTV CC regulator current
and 4) LTC3819 V IN current (including loading on the
differential amplifier output).
1) I 2 R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resistor,
and input and output capacitor ESR. In continuous mode
the average output current flows through L and R SENSE ,
but is “chopped” between the topside MOSFET and the
synchronous MOSFET. If the two MOSFETs have approxi-
mately the same R DS(ON) , then the resistance of one
MOSFET can simply be summed with the resistances of L,
R SENSE and ESR to obtain I 2 R losses. For example, if each
R DS(ON) = 10m ? , R L = 10m ? , and R SENSE = 5m ? , then the
total resistance is 25m ? . This results in losses ranging
from 2% to 8% as the output current increases from 3A to
15A per output stage for a 5V output, or a 3% to 12% loss
per output stage for a 3.3V output. Efficiency varies as the
inverse square of V OUT for the same external components
and output power level. The combined effects of increas-
ingly lower output voltages and higher currents required
by high performance digital systems is not doubling but
quadrupling the importance of loss terms in the switching
regulator system!
2) Transition losses apply only to the topside MOSFET(s),
and are significant only when operating at high input
voltages (typically 12V or greater). Transition losses can
be estimated from:
2 ? I O ( MAX ) ?
? 2 ?
3) INTV CC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high to
low again, a packet of charge dQ moves from INTV CC to
ground. The resulting dQ/dt is a current out of INTV CC that
is typically much larger than the control circuit current. In
continuous mode, I GATECHG = (Q T + Q B ), where Q T and Q B
are the gate charges of the topside and bottom side
MOSFETs.
Supplying INTV CC power through the EXTV CC switch input
from an output-derived source will scale the V IN current
required for the driver and control circuits by the ratio
(Duty Factor)/(Efficiency). For example, in a 20V to 5V
application, 10mA of INTV CC current results in approxi-
mately 3mA of V IN current. This reduces the mid-current
loss from 10% or more (if the driver was powered directly
from V IN ) to only a few percent.
4) The V IN current has two components: the first is the
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the differential
amplifier output. V IN current typically results in a small
(<0.1%) loss.
3819f
22
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