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LTC1159CS-3.3 数据表(PDF) 13 Page - Linear Technology

部件名 LTC1159CS-3.3
功能描述  High Efficiency Synchronous Step-Down Switching Regulators
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC1159CS-3.3 数据表(HTML) 13 Page - Linear Technology

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LTC1159/LTC1159-3.3/LTC1159-5
APPLICATIO S I FOR ATIO
LTC1159
SENSE+
SENSE
9
8
1000pF
R1
100
R2
100
L
RSENSE
+
COUT
R3
LTC1159 • F07
Figure 7. Suppressing Burst Mode Operation
low to high to low again, a packet of charge dQ moves
from VCC to ground. The resulting dQ/dt is a current out
of VCC which is typically much larger than the control
circuit current. In continuous mode, IGATECHG ≈ f (QP +
QN), where QP and QN are the gate charges of the two
MOSFETs.
By powering EXTVCC from an output-derived source, the
additional VIN current resulting from the driver and
control currents will be scaled by a factor of
(Duty Cycle)/(Efficiency). For example in a 20V to 5V
application, 10mA of VCC current results in approxi-
mately 3mA of VIN current. This reduces the mid-current
loss from 10% or more (if the driver was powered
directly from VIN) to only a few percent.
3. I
2R losses are easily predicted from the DC resistances
of the MOSFET, inductor, and current shunt. In con-
tinuous mode all of the output current flows through L
and RSENSE, but is “chopped” between the P-channel
and N-channel MOSFETs. If the two MOSFETs have
approximately the same RDS(ON), then the resistance of
one MOSFET can simply be summed with the resis-
tances of L and RSENSE to obtain I2R losses. For
example, if each RDS(ON) = 0.1Ω, RL = 0.15Ω, and
RSENSE = 0.05Ω, then the total resistance is 0.3Ω. This
results in losses ranging from 3% to 12% as the output
current increases from 0.5A to 2A. I2R losses cause the
efficiency to roll-off at high output currents.
4. Transition losses apply only to the P-channel MOSFET,
and only when operating at high input voltages (typi-
cally 20V or greater). Transition losses can be esti-
mated from:
Transition Loss
≈ 5(VIN)2(IMAX)(CRSS)(f)
Other losses including CINandCOUT ESRdissipativelosses,
Schottky conduction losses during dead time, and inductor
core losses, generally account for less than 2% total
additional loss.
Auxiliary Windings – Suppressing Burst Mode
Operation
The LTC1159 synchronous switch removes the normal
limitation that power must be drawn from the inductor
primary winding in order to extract power from auxiliary
windings. With synchronous switching, auxiliary out-
puts may be loaded without regard to the primary output
load, providing that the loop remains in continuous
mode operation.
Burst Mode operation can be suppressed at low output
currents with a simple external network which cancels the
0.025V minimum current comparator threshold. This tech-
nique is also useful for eliminating audible noise from
certain types of inductors in high current (IOUT > 5A)
applications when they are lightly loaded.
An external offset is put in series with the Sense pin to
subtract from the built-in 0.025V offset. An example of this
technique is shown in Figure 7. Two 100
Ω resistors are
inserted in series with the leads from the sense resistor.
With the addition of R3, a current is generated through R1
causing an offset of:
VOFFSET = VOUT
)
) R1
R1 + R3
If VOFFSET > 0.025V, the minimum threshold will be
cancelled and Burst Mode operation is prevented from
occurring. Since VOFFSET is constant, the maximum load
current is also decreased by the same offset. Thus, to get
back to the same IMAX, the value of the sense resistor must
be reduced:
RSENSE
m
75
IMAX
To prevent noise spikes from erroneously tripping the
current comparator, a 1000pF capacitor is needed across
the Senseand Sense+ pins.


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