LTC3835-1 Linear Technology, LTC3835-1 Datasheet - Page 19

• Manufacturer Part Number: LTC3835-1
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

www.datasheet4u.com
Effi ciency Considerations
The percent effi ciency 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 effi ciency and which change would
produce the most improvement. Percent effi ciency can
be expressed as:
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 LTC3835-1 circuits: 1) IC V
regulator current, 3) I
transition losses.
1. The V
2. INTV
3. I
APPLICATIONS INFORMATION
%Effi ciency = 100% – (L1 + L2 + L3 + ...)
DC supply current given in the Electrical Characteristics
table, which excludes MOSFET driver and control cur-
rents; the second is the current drawn from the 3.3V
linear regulator output. V
a small (<0.1%) loss.
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 INTV
rent out of INTV
control circuit current. In continuous mode, I
= f(Q
the topside and bottom side MOSFETs.
fuse (if used), MOSFET, inductor, current sense resis-
tor, and input and output capacitor ESR. In continuous
mode the average output current fl ows through L and
2
R losses are predicted from the DC resistances of the
CC
T
IN
+ Q
current is the sum of the MOSFET driver and
current has two components: the fi rst is the
CC
B
), where Q
to ground. The resulting dQ/dt is a cur-
CC
that is typically much larger than the
2
R losses, 4) Topside MOSFET
T
and Q
IN
current typically results in
B
are the gate charges of
IN
current, 2) INTV
GATECHG
CC
4. Transition losses apply only to the topside MOSFET(s),
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% effi ciency degradation in portable systems. It is
very important to include these “system” level losses
during the design phase. The internal battery and fuse
resistance losses can be minimized by making sure that
C
the switching frequency. A 25W supply will typically
require a minimum of 20μF to 40μF of capacitance hav-
ing a maximum of 20mΩ to 50mΩ of ESR. Other losses
including Schottky conduction losses during dead-time
and inductor core losses generally account for less than
2% total additional loss.
IN
R
and the synchronous MOSFET. If the two MOSFETs have
approximately the same R
of one MOSFET can simply be summed with the resis-
tances of L, R
example, if each R
= 10mΩ and R
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.
Effi ciency varies as the inverse square of V
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!
and become signifi cant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
has adequate charge storage and very low ESR at
SENSE
Transition Loss = (1.7) V
, but is “chopped” between the topside MOSFET
SENSE
ESR
DS(ON)
= 40mΩ (sum of both input and
and ESR to obtain I
= 30mΩ, R
DS(ON)
IN
2
I
O(MAX)
LTC3835-1
, then the resistance
L
= 50mΩ, R
C
2
RSS
R losses. For
OUT
f
19
for the
SENSE
38351fc