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HM1480 데이터시트(HTML) 8 Page - Shenzhen Huazhimei Semiconductor Co., Ltd

부품명 HM1480
상세내용  Input range synchronous buck controller
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제조사  HMSEMI [Shenzhen Huazhimei Semiconductor Co., Ltd]
홈페이지  http://www.hmsemi.com/
Logo HMSEMI - Shenzhen Huazhimei Semiconductor Co., Ltd

HM1480 데이터시트(HTML) 8 Page - Shenzhen Huazhimei Semiconductor Co., Ltd

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The selection of COUT is driven by the required effective series resistance (ESR).Typically, once the ESR
requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P)
requirement. The output ripple ?VOUT is determined by:
Where f = operating frequency, COUT = output capacitance and ?IL = ripple current in the inductor. For a
fixed output voltage, the output ripple is highest at maximum input voltage since ?IL increases with input
voltage.
Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In
the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies.
An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed
and tested for low ESR so they give the lowest ESR for a given volume.
Efficiency Considerations
The 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. 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, two main sources usually account for most of the
losses: VIN quiescent current and I2R losses. The VIN quiescent current loss dominates the efficiency
loss at very low load currents whereas the I2R 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 is switched from high to low to high again, a packet of charge ?Q moves from VIN
to ground. The resulting ?Q/?t is the current out of VIN that is typically larger than the DC bias current.
In continuous mode, IGATECHG = f (QT+QB) where QT and QB are the gate charges of the internal top and
bottom switches. Both the DC bias and gate charge losses are proportional to VIN and thus their effects
will be more pronounced at higher supply voltages.
2. I2R losses are calculated from the resistances of the internal switches, RSW and external inductor RL.
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 SW pin is a
function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = RDS(ON)TOP x
DC + RDS(ON)BOT x (1-DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the
Typical Performance Characteristics curves. Thus, to obtain I2R losses, simply add RSW to RL and
multiply the result by the square of the average output current. Other losses including CIN and COUT ESR
dissipative losses and inductor core losses generally account for less than 2% of the total loss.


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