LTC3448_15 Datasheet, PDF (9/20 Pages) Linear Technology – 1.5MHz/2.25MHz, 600mA Synchronous Step-Down Regulator with LDO Mode

U
OPERATIO (Refer to Functional Diagram)
1200
1000
V_OUT = 1.8V
800
V_OUT = 1.5V
600
V_OUT = 2.5V
400
200
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
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Figure 4. Maximum Output Current vs Input Voltage
Dropout Operation
As the input supply voltage decreases to a value approach-
ing the output voltage, the duty cycle increases toward the
maximum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one cycle
until it reaches 100% duty cycle. The output voltage will then
be determined by the input voltage minus the voltage drop
across the P-channel MOSFET and the inductor.
An important detail to remember is that at low input supply
voltages, the R_DS(ON) of the P-channel switch increases
LTC3448
(see Typical Performance Characteristics). Therefore, the
user should calculate the power dissipation when the
LTC3448 is used at 100% duty cycle with low input voltage
(See Thermal Considerations in the Applications Informa-
tion section).
Low Supply Operation
The LTC3448 will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 4 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing sub-harmonic oscilla-
tions at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. This normally
results in a reduction of maximum inductor peak current
for duty cycles >40%. However, the LTC3448 uses a
patent-pending scheme that counteracts this compensat-
ing ramp, which allows the maximum inductor peak
current to remain unaffected throughout all duty cycles.
APPLICATIO S I FOR ATIO
The basic LTC3448 application circuit is shown on the first
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of L followed by C_IN and C_OUT.
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1µH to 4.7µH. Its value is chosen based on the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. Higher V_IN or V_OUT also increases the ripple
current as shown in equation 1. A reasonable starting point
for setting ripple current is ∆I_L = 240mA (40% of 600mA).
( )( ) ^{∆IL =}
1
fL
V_OUT
⎛
⎝⎜^1−
V_OUT
V_IN
⎞
⎠⎟
(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA rated
inductor should be enough for most applications (600mA
+ 120mA). For better efficiency, choose a low DC-resis-
tance inductor.
If the LTC3448 is to be used in auto LDO mode, inductor
values less than 1µH should not be used.
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