LTC3543_15 Datasheet, PDF (10/20 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

LTC3543
OPERATION
An important detail to remember is that at low input supply
voltages, the R_DS(ON) of the P-channel switch increases
(see the Typical Performance Characteristics section).
Therefore, the user should calculate the power dissipation
when the LTC3543 is used at 100% duty cycle with low
input voltage (see Thermal Considerations in the Applica-
tions Information section).
Low Supply Operation
The LTC3543 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 reduc-
tion 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-
frequency architectures by preventing subharmon-
ic oscillations 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 LTC3543
uses a patent pending scheme that counteracts
this compensating ramp, allowing the maximum induc-
tor peak current to remain unaffected throughout all
duty cycles.
1500
1400
1300
1200
1100
V_OUT = 1.2V
1000
900
800
V_OUT = 1.5V
700
600
500
2.5 3.0 3.5 4.0 4.5 5.0 5.5
INPUT VOLTAGE (V)
3543 F02
Figure 2. Maximum Output Current vs Input Voltage (V_IN)
3543fa
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