LTC1872BES6 LINER [Linear Technology], LTC1872BES6 Datasheet - Page 9

• Manufacturer Part Number: LTC1872BES6
• Description: Constant Frequency Current Mode Step-Up DC/DC Controller in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
Efficiency Considerations
The 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. Efficiency can be expressed as:
where 1, 2, 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 LTC1872B circuits: 1) LTC1872B DC bias cur-
rent, 2) MOSFET gate charge current, 3) I
voltage drop of the output diode.
1. The V
2. MOSFET gate charge current results from switching
3. I
4. The output diode is a major source of power loss at
Efficiency = 100% – ( 1 + 2 + 3 + ...)
electrical characteristics, that excludes MOSFET driver
and control currents. V
which increases with V
the gate capacitance of the power MOSFET. Each time
a MOSFET gate is switched from low to high to low
again, a packet of charge, dQ, moves from V
ground. The resulting dQ/dt is a current out of V
which is typically much larger than the contoller’s DC
supply current. In continuous mode, I
MOSFET, inductor and current sense resistor. The
MOSFET R
average output current squared can be summed with
I
sense resistor.
high currents. The diode loss is calculated by multiply-
ing the forward voltage by the load current.
2
2
R losses are predicted from the DC resistances of the
R losses in the inductor ESR in series with the current
IN
current is the DC supply current, given in the
DS(ON)
U
multiplied by duty cycle times the
INFORMATION
IN
U
IN
current results in a small loss
.
W
2
GATECHG
R losses and 4)
U
= f(Qp).
IN
to
IN
5. Transition losses apply to the external MOSFET and
Other losses, including C
losses, and inductor core losses, generally account for
less than 2% total additional loss.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1872B. These items are illustrated graphically in the
layout diagram in Figure 6. Check the following in your
layout:
1. The Schottky diode should be closely connected
2. The (+) plate of C
3. The input decoupling capacitor (0.1 F) should be
4. Connect the end of R
5. The trace from SENSE
6. Keep the switching node NGATE away from sensitive
7. The V
increase at higher operating frequencies and input
voltages. Transition losses can be estimated from:
Transition Loss = 2(V
between the output capacitor and the drain of the
external MOSFET.
resistor as closely as possible. This capacitor provides
AC current to the inductor.
connected closely between V
(Pin 2).
possible. The V
comparator.
should be kept short. The trace should connect close
to R
small signal nodes.
resistors. The resistive divider R1 and R2 must be
connected between the (+) plate of C
ground.
SENSE
FB
pin should connect directly to the feedback
.
IN
pin is the SENSE
IN
SENSE
IN
should connect to the sense
–
IN
)
2
(Pin 4) to the Sense resistor
I
IN(MAX)
and C
as close to V
IN
OUT
(Pin 5) and ground
C
RSS
LTC1872B
+
ESR dissipative
(f)
OUT
of the current
IN
and signal
(Pin 5) as
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