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參數(shù)資料
| 型號： | LTC3890IGN-1#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, PDSO28 |
| 封裝： | LEAD FREE, PLASTIC, SSOP-28 |
| 文件頁數(shù)： | 17/36頁 |
| 文件大?。?/td> | 498K |
| 代理商： | LTC3890IGN-1#TRPBF |
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LTC3890-1
24
38901f
APPLICATIONS INFORMATION
The minimum on-time for the LTC3890-1 is approximately
90ns. However, as the peak sense voltage decreases the
minimum on-time gradually increases up to about TBDns.
This is of particular concern in forced continuous applica-
tions with low ripple current at light loads. If the duty cycle
drops below the minimum on-time limit in this situation,
a signicant amount of cycle skipping can occur with
correspondingly larger current and voltage ripple.
Efciency Considerations
The percent efciency 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 efciency and which change would
produce the most improvement. Percent efciency can
be expressed as:
%Efciency = 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 LTC3890-1 circuits: 1) IC VIN current, 2) INTVCC
regulator current, 3) I2R losses, 4) topside MOSFET
transition losses.
1. The VIN current is the DC supply current given in the
Electrical Characteristics table, which excludes MOSFET
driver and control currents. VIN current typically results
in a small (<0.1%) loss.
2. INTVCC 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 INTVCC to ground. The resulting dQ/dt is a current
out of INTVCC that is typically much larger than the
control circuit current. In continuous mode, IGATECHG
= f(QT + QB), where QT and QB are the gate charges of
the topside and bottom side MOSFETs.
Supplying INTVCCfromanoutput-derivedsourcepower
through EXTVCC will scale the VIN current required
for the driver and control circuits by a factor of (Duty
Cycle)/(Efciency). For example, in a 20V to 5V applica-
tion, 10mA of INTVCC current results in approximately
2.5mA of VIN current. This reduces the midcurrent loss
from 10% or more (if the driver was powered directly
from VIN) to only a few percent.
3. I2R 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
RSENSE, but is chopped between the topside MOSFET
and the synchronous MOSFET. If the two MOSFETs
have approximately the same RDS(ON), then the resis-
tance of one MOSFET can simply be summed with the
resistances of L, RSENSE and ESR to obtain I2R losses.
For example, if each RDS(ON) = 30mΩ, RL = 50mΩ,
RSENSE = 10mΩ and RESR = 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.
Efciency varies as the inverse square of VOUT 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 topside MOSFET(s),
and become signicant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
Transition Loss = (1.7) VIN 2 IO(MAX) CRSS f
Other hidden losses such as copper trace and internal
battery resistances can account for an additional 5%
to 10% efciency degradation in portable systems. It
is very important to include these system level losses
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