LTC1159 Linear Technology, LTC1159 Datasheet - Page 13

• Manufacturer Part Number: LTC1159
• Description: High Efficiency Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
3. I
4. Transition losses apply only to the P-channel MOSFET,
Other losses including C
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
low to high to low again, a packet of charge dQ moves
from V
of V
circuit current. In continuous mode, I
Q
MOSFETs.
By powering EXTV
additional V
control currents will be scaled by a factor of
(Duty Cycle)/(Efficiency). For example in a 20V to 5V
application, 10mA of V
mately 3mA of V
loss from 10% or more (if the driver was powered
directly from V
of the MOSFET, inductor, and current shunt. In con-
tinuous mode all of the output current flows through L
and R
and N-channel MOSFETs. If the two MOSFETs have
approximately the same R
one MOSFET can simply be summed with the resis-
tances of L and R
example, if each R
R
results in losses ranging from 3% to 12% as the output
current increases from 0.5A to 2A. I
efficiency to roll-off at high output currents.
and only when operating at high input voltages (typi-
cally 20V or greater). Transition losses can be esti-
mated from:
Transition Loss 5(V
2
N
SENSE
R losses are easily predicted from the DC resistances
), where Q
CC
SENSE
CC
which is typically much larger than the control
= 0.05 , then the total resistance is 0.3 . This
to ground. The resulting dQ/dt is a current out
, but is “chopped” between the P-channel
IN
P
IN
and Q
current resulting from the driver and
IN
) to only a few percent.
U U
CC
current. This reduces the mid-current
DS(ON)
SENSE
from an output-derived source, the
IN
N
IN
and C
are the gate charges of the two
)
CC
2
(I
DS(ON)
MAX
current results in approxi-
to obtain I
= 0.1 , R
OUT
)(C
ESR dissipative losses,
, then the resistance of
W
RSS
2
R losses cause the
GATECHG
)(f)
L
2
R losses. For
= 0.15 , and
U
f (Q
P
+
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 (I
applications when they are lightly loaded.
An external offset is put in series with the Sense
subtract from the built-in 0.025V offset. An example of this
technique is shown in Figure 7. Two 100 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:
If V
cancelled and Burst Mode operation is prevented from
occurring. Since V
current is also decreased by the same offset. Thus, to get
back to the same I
be reduced:
To prevent noise spikes from erroneously tripping the
current comparator, a 1000pF capacitor is needed across
the Sense
LTC1159/LTC1159-3.3/LTC1159-5
V
R
LTC1159
OFFSET
OFFSET
SENSE
SENSE
SENSE
Figure 7. Suppressing Burst Mode Operation
–
+
–
and Sense
> 0.025V, the minimum threshold will be
= V
9
8
I
MAX
75
OUT
MAX
OFFSET
1000pF
m
R1 + R3
, the value of the sense resistor must
+
R1
pins.
is constant, the maximum load
R3
L
100
100
R2
R1
R
SENSE
LTC1159 • F07
OUT
–
13
> 5A)
pin to
+
C
OUT