LTC1871-7
19
18717fd
applicaTions inForMaTion
During this inductor charging interval, the output capacitor
must supply the load current and a significant droop in
the output voltage can occur. Generally, it is a good idea
to choose a value of inductor ∆I
L
between 25% and 40%
of I
IN(MAX)
. The alternative is to either increase the value
of the output capacitor or disable Burst Mode operation
using the MODE/SYNC pin.
Burst Mode operation can be defeated by connecting the
MODE/SYNC pin to a high logic-level voltage (either with
a control input or by connecting this pin to INTV
CC
). In
this mode, the burst clamp is removed, and the chip can
operate at constant frequency from continuous conduction
mode (CCM) at full load, down into deep discontinuous
conduction mode (DCM) at light load. Prior to skipping
pulses at very light load (i.e., <5% of full load), the control-
ler will operate with a minimum switch on-time in DCM.
Pulse skipping prevents a loss of control of the output at
very light loads and reduces output voltage ripple.
Efficiency Considerations
The efficiency of a switching regulator is equal to the out-
put power divided by the input power (¥100%). Percent
efficiency can be expressed as:
% Efficiency = 100% – (L1 + L2 + L3 + …),
where L1, L2, etc. are the individual loss components as a
percentage of the input power. It is often useful to analyze
individual losses to determine what is limiting the efficiency
and which change would produce the most improvement.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for the majority
of the losses in LTC1871-7 application circuits:
1. The supply current into V
IN
. The V
IN
current is the sum
of the DC supply current I
Q
(given in the Electrical Char-
acteristics) and the MOSFET driver and control currents.
The DC supply current into the V
IN
pin is typically about
650µA and represents a small power loss (much less
than 1%) that increases with V
IN
. The driver current
results from switching the gate capacitance of the power
MOSFET; this current is typically much larger than the
DC current. Each time the MOSFET is switched on and
then off, a packet of gate charge Q
G
is transferred from
INTV
CC
to ground. The resulting dQ/dt is a current that
must be supplied to the INTV
CC
capacitor through the
V
IN
pin by an external supply. If the IC is operating in
CCM:
I
Q(TOT)
≈ I
Q
= f • Q
G
P
IC
= V
IN
• (I
Q
+ f • Q
G
)
2. Power MOSFET switching and conduction losses:
P
FET
=
I
O(MAX)
1– D
MAX
2
•R
DS(ON)
•D
MAX
• ρ
T
+ k • V
O
2
•
I
O(MAX)
1– D
• C
RSS
• f
3. The I
2
R losses in the sense resistor can be calculated
almost by inspection.
P
R(SENSE)
=
I
O(MAX)
1– D
MAX
2
• R
SENSE
• D
MAX
4. The losses in the inductor are simply the DC input cur-
rent squared times the winding resistance. Expressing
this loss as a function of the output current yields:
P
R(WINDING)
=
I
O(MAX)
1– D
MAX
2
• R
W
5. Losses in the boost diode. The power dissipation in the
boost diode is:
P
DIODE
= I
O(MAX)
• V
D
The boost diode can be a major source of power loss
in a boost converter. For 13.2V input, 42V output at
1.5A example given in Figure 9, a Schottky diode with
a 0.4V forward voltage would dissipate 600mW, which
represents about 1% of the input power. Diode losses
can become significant at low output voltages where
the forward voltage is a significant percentage of the
output voltage.
6. Other losses, including C
IN
and C
O
ESR dissipation and
inductor core losses, generally account for less than
2% of the total losses.
