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| 型号： | LTC3859EUHF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 30/42页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM CM 38-QFN |
| 标准包装： | 52 |
| PWM 型： | 电流模式，Burst Mode? |
| 输出数： | 3 |
| 频率 - 最大： | 850kHz |
| 占空比： | 100% |
| 电源电压： | 4.5 V ~ 38 V |
| 降压： | 是 |
| 升压： | 是 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 38-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3859
APPLICATIONS INFORMATION
Ef?ciency Considerations
The percent ef?ciency 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 ef?ciency and which change would
produce the most improvement. Percent ef?ciency can
be expressed as:
%Ef?ciency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3859 circuits: 1) IC V IN current, 2) INTV CC
regulator current, 3) I 2 R losses, 4) Topside MOSFET
transition losses.
1. The V IN current is the DC supply current given in the
Electrical Characteristics table, which excludes MOSFET
driver and control currents. V IN current typically results
in a small (<0.1%) loss.
2. 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
= f(Q T + Q B ), where Q T and Q B are the gate charges of
the topside and bottom side MOSFETs.
Supplying INTV CC from an output-derived source power
through EXTV CC will scale the V IN current required
for the driver and control circuits by a factor of (Duty
Cycle)/(Ef?ciency). For example, in a 20V to 5V applica-
tion, 10mA of INTV CC current results in approximately
2.5mA 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.
R SENSE , but is “chopped” between the topside MOSFET
and the synchronous MOSFET. If the two MOSFETs have
approximately the same R DS(ON) , then the resistance
of one MOSFET can simply be summed with the resis-
tances of L, R SENSE and ESR to obtain I 2 R losses. For
example, if each R DS(ON) = 30mΩ, R L = 50mΩ, R SENSE
= 10mΩ and R ESR = 40mΩ (sum of both input and
output capacitance losses), then the total resistance
is 130mΩ. This results in losses ranging from 3% to
13% as the output current increases from 1A to 5A for
a 5V output, or a 4% to 20% loss for a 3.3V output.
Ef?ciency varies as the inverse square of V OUT for the
same external components and output power level. The
combined effects of increasingly 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!
4. Transition losses apply only to the top MOSFET(s) (bot-
tom MOSFET for the boost), and become signi?cant only
when operating at high input voltages (typically 15V or
greater). Transition losses can be estimated from:
Transition Loss = (1.7)V IN2 ? I O(MAX) ? C RSS ? f
Other hidden losses such as copper trace and internal
battery resistances can account for an additional 5%
to 10% ef?ciency degradation in portable systems. It is
very important to include these “system” level losses
during the design phase. The internal battery and fuse
resistance losses can be minimized by making sure that
C IN has adequate charge storage and very low ESR at
the switching frequency. A 25W supply will typically
require a minimum of 20μF to 40μF of capacitance
having a maximum of 20mΩ to 50mΩ of ESR. The
LTC3859 2-phase architecture typically halves this input
capacitance requirement over competing solutions.
Other losses including Schottky conduction losses
during dead-time and inductor core losses generally
account for less than 2% total additional loss.
Checking Transient Response
3.
I 2 R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resis-
tor, and input and output capacitor ESR. In continuous
mode the average output current ?ows through L and
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, V OUT shifts by an
3859fa
30
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