14
LTC1266
LTC1266 -3.3/LTC1266 -5
15nC. This results in I
GATECHG
= 6mA in 200kHz continu-
ous operation for a 2% to 3% typical mid-current loss with
V
IN
= 5V.
Note that the gate charge loss increases directly with
both input voltage and operating frequency. This is the
principal reason why the highest efficiency circuits oper-
ate at moderate frequencies. Furthermore, it argues against
using larger MOSFETs than necessary to control I
2
R
losses, since overkill can cost efficiency as well as money!
3. I
2
R losses are easily predicted from the DC resistances
of the MOSFET, inductor and current shunt. In continuous
mode the average output current flows through L and
R
SENSE
, but is “chopped” between the topside and bot-
tom-side 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)
= 0.05Ω, R
L
= 0.05Ω and R
SENSE
= 0.02Ω, then
the total resistance is 0.12Ω. This results in losses ranging
from 3.5% to 15% as the output current increases from 1A
to 5A. I
2
R losses cause the efficiency to roll off at high
output currents.
Figure 8 shows how the efficiency losses in a typical
LTC1266 series regulator end up being apportioned. The
gate charge loss is responsible for the majority of the
efficiency lost in the mid-current region. If Burst Mode
operation was not employed at low currents, the gate
charge loss alone would cause efficiency to drop to
unacceptable levels (see Figure 7). With Burst Mode
discharge C
OUT
until the regulator loop adapts to the
current change and returns V
OUT
to its steady-state value.
During this recovery time V
OUT
can be monitored for
overshoot or ringing which would indicate a stability
problem. The Pin 7 external components shown in the
Figure 1 circuit will prove adequate compensation for
most applications.
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 percent-
age of input power. (For high efficiency circuits, only small
errors are incurred by expressing losses as a percentage
of output power).
Although all dissipative elements in the circuit produce
losses, three main sources usually account for most of the
losses in LTC1266 series circuits: 1) LTC1266 DC bias
current, 2) MOSFET gate charge current and 3) I
2
R losses.
1. The DC supply current is the current which flows into
V
IN
(Pin 2). For V
IN
= 10V the LTC1266 DC supply current
is 170µA for no load, and increases proportionally with
load up to a constant 2.1mA after the LTC1266 series has
entered continuous mode. Because the DC bias current is
drawn from V
IN
, the resulting loss increases with input
voltage. For V
IN
= 5V the DC bias losses are generally less
than 1% for load currents over 30mA. However, at very
low load currents the DC bias current accounts for nearly
all of the loss.
2. MOSFET gate charge 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 Power V
IN
to ground. The
resulting dQ/dt is a current flowing into Power V
IN
(Pin 5)
which is typically much larger than the DC supply current.
In continuous mode, I
GATECHG
= f (Q
N
+ Q
P
). The typical
gate charge for a 0.05Ω N-channel power MOSFET is
Figure 8. Efficiency Loss
I
OUT
(A)
0.01
EFFICIENCY/LOSS (%)
90
95
1
1266 F08
85
80
0.03
0.1
0.3
5
100
GATE CHARGE
LTC1266 I
Q
I
2
R
APPLICATIO S I FOR ATIO
WUUU
