LTC1626 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1626' PDF : 12 Pages View PDF
LTC1626
APPLICATIONS INFORMATION
inductor current should be continuous during burst peri-
ods. Hence, the peak-to-peak inductor ripple current must
not exceed 25mV/RSENSE.
To account for light load conditions, the IOUT(MAX) is then
given by:
IOUT(MAX) = 150mV/RSENSE – 25mV/2RSENSE (A)
= 137.5mV/RSENSE (A)
Solving for RSENSE and allowing a margin of variations in
the LTC1626 and external component values yields:
RSENSE = 100mV/IOUT(MAX) (Ω)
The LTC1626 switch is capable of supplying a maximum
of 1.2A of output current. Therefore, the minimum value
of RSENSE that can be used is 0.083Ω. A graph for
selecting RSENSE versus maximum output current is given
in Figure 2.
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8 1.0
MAXIMUM OUTPUT CURRENT (A)
1626 F02
Figure 2. Selecting RSENSE
During a short circuit of the regulator output to ground, the
peak current is determined by:
ISC = 150mV/RSENSE (A)
In this condition, the LTC1626 automatically extends the
off-time period of the P-channel MOSFET switch to allow
the inductor current to decay far enough to prevent any
current buildup. The resulting ripple current causes the
average current to be approximately IOUT(MAX).
Operating Frequency Considerations
For most applications, the LTC1626 should be operated in
the 100kHz to 300kHz range. This range can be extended,
however, up to 600kHz, to accommodate smaller size/
valued inductors, such as low profile types, with a slight
decrease in efficiency due to gate charge losses. Some
experimentation may be required to determine the opti-
mum operating frequency for a particular set of external
components and operating conditions.
CT and L Selection
The value of CT is calculated from the desired continuous
mode operating frequency:
( )(( )( ) )( ) ( ) CT =
VIN + VD
VIN − VOUT
3300 VIN − VBE
fO
F
where VD is the drop across the Schottky diode D1 and VBE
is a base-emitter voltage drop (0.6V).
The complete expression for operating frequency is given
by:
( ) fO
≈
1
tOFF
VIN − VOUT
VIN + VD
Hz
where:
( )( )( ) ( ) tOFF = 3300 CT VIN − VBE sec
Figure 3 is a graph of operating frequency versus power
supply voltage for the 2.5V regulator circuit shown in
Figure 1 (CT = 270pF). Note that the frequency is relatively
constant with supply voltage but drops as the supply
voltage approaches the regulated output voltage.
To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
RSENSE of peak-to-peak ripple current. This results in the
following expression for L:
( ) ( )( )( ) ( ) L ≥ 5.2 105 RSENSE CT VREG H
Using an inductance smaller than the above value will
result in inductor current being discontinuous. As a con-
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