13
LTC1159
LTC1159-3.3/LTC1159-5
the same process as in conventional applications, using
either the internal divider (LTC1159-3.3, LTC1159-5) or an
external divider with the adjustable version.
Figure 15 in the Typical Applications shows a synchronous
12V to –12V converter that can supply up to 1A with better
than 85% efficiency. By grounding the EXTV
CC
pin in the
Figure 15 circuit, the entire 12V output voltage is placed
across the driver and control circuits since the LTC1159
ground pins are at –12V. During start-up or short-circuit
conditions, operating power is supplied by the internal
4.5V regulator. The shutdown signal is level-shifted to the
negative output rail by Q3, and Q4 ensures that Q1 and Q2
remain off during the entire shutdown sequence.
Efficiency Considerations
The percent 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. Percent 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, four main sources usually account for most of the
losses in LTC1159 circuits: 1) LTC1159 V
IN
current, 2)
LTC1159 V
CC
current, 3) I
2
R losses and 4) P-channel
transition losses.
1. LTC1159 V
IN
current is the DC supply current given in
the electrical characteristics which excludes MOSFET driver
and control currents. V
IN
current results in a small (<1%)
loss which increases with V
IN
.
2. LTC1159 V
CC
current is the sum of the MOSFET driver
and control circuit 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 V
CC
to ground. The resulting dQ/dt is a current out of V
CC
which
is typically much larger than the control circuit current. In
continuous mode, I
GATECHG
≈ f (Q
P
+ Q
N
), where Q
P
and Q
N
are the gate charges of the two MOSFETs.
By powering EXTV
CC
from an output-derived source, the
additional V
IN
current resulting from the driver and control
currents will be scaled by a factor of (Duty Cycle)/(Effi-
ciency). For example in a 20V to 5V application, 10mA of
V
CC
current results in approximately 3mA 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.
3. I
2
R losses are easily predicted from the DC resistances
of the MOSFET, inductor and current shunt. In continuous
mode all of the output current flows through L and
R
SENSE
, but is “chopped” between the P-channel and
N-channel MOSFETs. If the two MOSFETs have approxi-
mately the same R
DS(ON)
, then the resistance of one
MOSFET can simply be summed with the resistances of L
and R
SENSE
to obtain I
2
R losses. For example, if each
R
DS(ON)
= 0.1Ω, R
L
= 0.15Ω, and R
SENSE
= 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. I
2
R 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 (typically
20V or greater). Transition losses can be estimated from:
Transition Loss ≈ 5(V
IN
)
2
(I
MAX
)(C
RSS
)(f)
Other losses including C
IN
and C
OUT
ESR dissipative losses,
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 that cancels the
0.025V minimum current comparator threshold. This tech-
nique is also useful for eliminating audible noise from
APPLICATIO S I FOR ATIO
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