ltc1622 Linear Technology Corporation, ltc1622 Datasheet - Page 10

• Manufacturer Part Number: ltc1622
• Description: Low Input Voltage Current Mode Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC1622
the maximum current sense voltage that sets the maxi-
mum output current.
Setting Output Voltage
The LTC1622 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 output voltage. The regulated
output voltage is determined by:
For most applications, a 30k resistor is suggested for R1.
To prevent stray pickup, an optional 100pF capacitor is
suggested across R1 located close to LTC1622.
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
10
V
OUT
101
100
Figure 3. Line Regulation of V
0 8 1
99
98
97
96
95
2.0
Figure 4. Setting Output Voltage
LTC1622
2.2
R
R
U
V
2
FB
1
INPUT VOLTAGE (V)
3
V
V
2.4
INFORMATION
REF
ITH
U
100pF
2.6
R2
R1
W
2.8
REF
and V
1622 F03
1622 F04
V
OUT
3.0
ITH
U
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 LTC1622 circuits: 1) LTC1622 DC bias current,
2) MOSFET gate charge current, 3) I
drop of the output diode and 5) transition losses.
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
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
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 resistance of the inductor to obtain I
high currents and gets worse at high input voltages.
The diode loss is calculated by multiplying the forward
voltage drop 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 with higher operating frequencies and input
voltages. Transition losses can be estimated from:
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
current results in a small loss
.
= f(Qp).
2
R losses, 4) voltage
IN
IN
to ground.
2
R losses.
which is
DS(ON)