LTC1772 Linear Technology, LTC1772 Datasheet - Page 9

• Manufacturer Part Number: LTC1772
• Description: Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23
• Manufacturer: Linear Technology

APPLICATIONS
Setting Output Voltage
The LTC1772 develops a 0.8V reference voltage between
the feedback (Pin 3) terminal and ground (see Figure 4). 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, a 100pF capacitor is suggested
across R1 located close to LTC1772.
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 LTC1772 circuits: 1) LTC1772 DC bias current,
2) MOSFET gate charge current, 3) I
voltage drop of the output diode.
Efficiency = 100% – ( 1 + 2 + 3 + ...)
V
OUT
0 8 1
.
Figure 4. Setting Output Voltage
LTC1772
R
U
R
V
FB
2
1
3
INFORMATION
U
100pF
R2
R1
W
2
R losses and 4)
1772 F04
V
OUT
U
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 external 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 power MOSFET. Each time
a MOSFET gate is switched from low to high to low
again, a packet of charge dQ moves from V
The resulting dQ/dt is a current out of V
typically much larger than the DC supply current. In
continuous mode, I
MOSFET, inductor and current shunt. In continuous
mode the average output current flows through L but
is “chopped” between the P-channel MOSFET in series
with R
plus R
with the resistances of L and R
losses.
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
R losses are predicted from the DC resistances of the
IN
SENSE
SENSE
current is the DC supply current, given in the
and the output diode. The MOSFET R
multiplied by duty cycle can be summed
GATECHG
IN
IN
IN
IN
)
2
current results in a small loss
.
I
and C
O(MAX)
= f(Qp).
OUT
C
SENSE
RSS
ESR dissipative
(f)
LTC1772
to obtain I
IN
IN
to ground.
which is
DS(ON)
9
2
R