LTC1779 LINER [Linear Technology], LTC1779 Datasheet - Page 9

• Manufacturer Part Number: LTC1779
• Description: 250mA Current Mode Step-Down DC/DC Converter in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Setting Output Voltage
The LTC1779 develops a 0.8V reference voltage between
the feedback (Pin 3) terminal and ground (see Figure 5). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
For most applications, an 80k resistor is suggested for R1.
To prevent stray pickup, locate resistors R1 and R2 close
to LTC1779.
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.
Efficiency = 100% – ( 1 + 2 + 3 + ...)
V
OUT
105
100
Figure 4. Line Regulation of V
0 8 1
95
90
85
80
75
.
2.0
Figure 5. Setting Output Voltage
LTC1779
2.2
R
U
R
2
1
INPUT VOLTAGE (V)
V
FB
V
V
2.4
REF
ITH
U
3
2.6
R2
R1
W
2.8
REF
1779 F05
V
OUT
and V
1779 F04
3.0
ITH
U
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1779 circuits: 1) LTC1779 DC bias current,
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
5. Transition losses apply to the internal MOSFET and
Other losses including C
losses, and inductor core losses, generally account for
less than 2% total additional loss.
electrical characteristics, that excludes MOSFET driver
and control currents. V
which increases with V
the gate capacitance of the internal power MOSFET.
Each time the MOSFET gate is switched from low to
high to low again, a packet of charge dQ moves from
V
V
current. In continuous mode, I
internal MOSFET, inductor and current shunt. In con-
tinuous mode the average output current flows through
L but is “chopped” between the internal P-channel
MOSFET in series with R
The MOSFET R
cycle can be summed with the resistances of L and
R
high currents and gets worse at high input voltages.
The diode loss is calculated by multiplying the forward
voltage times the diode duty cycle multiplied by the
load current. For example, assuming a duty cycle of
50% with a Schottky diode forward voltage drop of
0.4V, the loss increases from 0.5% to 8% as the load
current increases from 0.5A to 2A.
increase at higher operating frequencies and input
voltages. Transition losses can be estimated from:
Transition Loss = 2(V
2
IN
IN
SENSE
R losses are predicted from the DC resistances of the
to ground. The resulting dQ/dt is a current out of
which is typically much larger than the DC supply
IN
to obtain I
current is the DC supply current, given in the
DS(ON)
2
R losses.
IN
plus R
IN
IN
IN
)
2
current results in a small loss
.
SENSE
I
and C
O(MAX)
SENSE
GATECHG
and the output diode.
OUT
C
RSS
multiplied by duty
2
R losses and 4)
ESR dissipative
(f)
LTC1779
= f(Qp).
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