LTC3408_15 Datasheet, PDF (7/12 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator with Bypass Transistor

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OPERATIO (Refer to Functional Diagram)
off and on the bypass P-channel MOSFET with a frequency
of approximately 50kHz to 100kHz at 1.6A peak current.
This will continue until the short is removed. While the
bypass P-channel MOSFET is pulsing intermittently, the
inherent current limit of the step-down regulator limits its
peak current to about 1A.
Dropout Operation
If the reference voltage would cause V_OUT to exceed V_IN,
the LTC3408 enters dropout operation. During dropout,
the main switch remains on continuously and operates at
100% duty cycle. If the voltage at REF is less than 1.2V, the
bypass P-channel MOSFET will stay off even in dropout
operation. The output voltage is then determined by the
input voltage minus the voltage drop across the main switch
and the inductor. If the voltage at REF is greater than 1.2V,
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)
3408 F02
Figure 2. Maximum Output Current vs Input Voltage
LTC3408
but less than V_IN/3, the bypass P-channel MOSFET will be
on, but the main switch will be off. For best performance
and lowest voltage drop from V_IN to V_OUT, always ensure
that the REF voltage is greater than both 1.2V and V_IN/3.
An important detail to remember is that at low input
supply voltages, the R_DS(ON) of the P-channel switch
increases (see Typical Performance Characteristics).
Therefore, the user should calculate the power dissipa-
tion when the LTC3408 is used at 100% duty cycle with
low input voltage (See Thermal Considerations in the
Applications Information section).
Low Supply Operation
The LTC3408 will operate with input supply voltages as
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 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 subharmonic 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%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles > 40%. However, the LTC3408 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 LTC3408 application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection 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 4µH to 6µH. 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. As Equation 1 shows, a greater difference be-
tween V_IN and V_OUT produces a larger ripple current.
Where these voltages are subject to change, the highest
V_IN and lowest V_OUT will determine the maximum ripple
current. A reasonable starting point for setting ripple
current is I_L = 120mA (20% of the maximum load, 600mA).
∆I_L
=
1
(f)(L)
V_OUT

^1–
V_OUT
V_IN


(1)
3408f
7