Datasheet LTC3448 (Analog Devices) - 9

LTC3448
U OPERATIO (Refer to Functional Diagram)
1200 (see Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the 1000 LTC3448 is used at 100% duty cycle with low input voltage VOUT = 1.8V 800 V (See Thermal Considerations in the Applications Informa- OUT = 2.5V V tion section). OUT = 1.5V 600
Low Supply Operation
400 The LTC3448 will operate with input supply voltages as 200 MAXIMUM OUTPUT CURRENT (mA) low as 2.5V, but the maximum allowable output current is 0 reduced at this low voltage. Figure 4 shows the reduction 2.5 3.0 3.5 4.0 4.5 5.0 5.5 in the maximum output current as a function of input SUPPLY VOLTAGE (V) 3448 F04 voltage for various output voltages.
Figure 4. Maximum Output Current vs Input Voltage Slope Compensation and Inductor Peak Current Dropout Operation
Slope compensation provides stability in constant fre- quency architectures by preventing sub-harmonic oscilla- As the input supply voltage decreases to a value approach- tions at high duty cycles. It is accomplished internally by ing the output voltage, the duty cycle increases toward the adding a compensating ramp to the inductor current maximum on-time. Further reduction of the supply voltage signal at duty cycles in excess of 40%. This normally forces the main switch to remain on for more than one cycle results in a reduction of maximum inductor peak current until it reaches 100% duty cycle. The output voltage will then for duty cycles >40%. However, the LTC3448 uses a be determined by the input voltage minus the voltage drop patent-pending scheme that counteracts this compensat- across the P-channel MOSFET and the inductor. ing ramp, which allows the maximum inductor peak An important detail to remember is that at low input supply current to remain unaffected throughout all duty cycles. voltages, the RDS(ON) of the P-channel switch increases
U U W U APPLICATIO S I FOR ATIO
The basic LTC3448 application circuit is shown on the first 1 ⎛ V ⎞ page of this data sheet. External component selection is ∆I OUT L = VOUT 1 (1) driven by the load requirement and begins with the selec- (f)( ) − L ⎝⎜ VIN ⎠⎟ tion of L followed by CIN and COUT. The DC current rating of the inductor should be at least
Inductor Selection
equal to the maximum load current plus half the ripple For most applications, the value of the inductor will fall in current to prevent core saturation. Thus, a 720mA rated the range of 1µH to 4.7µH. Its value is chosen based on the inductor should be enough for most applications (600mA desired ripple current. Large value inductors lower ripple + 120mA). For better efficiency, choose a low DC-resis- current and small value inductors result in higher ripple tance inductor. currents. Higher VIN or VOUT also increases the ripple If the LTC3448 is to be used in auto LDO mode, inductor current as shown in equation 1. A reasonable starting point values less than 1µH should not be used. for setting ripple current is ∆IL = 240mA (40% of 600mA). 3448f 9