Datasheet LTC1877 (Analog Devices) - 9

LTC1877
OPERATION Dropout Operation
Another important detail to remember is that at low input supply voltages, the R When the input supply voltage decreases toward the output DS(ON) of the P-channel switch in- creases. Therefore, the user should calculate the power voltage, the duty cycle increases toward the maximum dissipation when the LTC1877 is used at 100% duty cycle on-time. Further reduction of the supply voltage forces the with a low input voltage (see Thermal Considerations in main switch to remain on for more than one cycle until it the Applications Information section). reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop
Slope Compensation and Inductor Peak Current
across the internal P-channel MOSFET and the inductor. Slope compensation provides stability in constant-fre-
Low Supply Operation
quency architectures by preventing subharmonic oscilla- tions at high duty cycles. It is accomplished internally by The LTC1877 is designed to operate down to an input supply adding a compensating ramp to the inductor current signal voltage of 2.65V although the maximum allowable output at duty cycles in excess of 40%. As a result, the maximum current is reduced at this low voltage. Figure 1 shows the inductor peak current is reduced for duty cycles >40%. reduction in the maximum output current as a function of This is shown in the decrease of the inductor peak current input voltage for various output voltages. as a function of duty cycle graph in Figure 2. 1200 1100 VIN = 5V VOUT = 2.5V 1000 1000 VOUT = 1.5V 800 VOUT = 5V 900 600 VOUT = 3.3V 800 400 MAX OUTPUT CURRENT (mA) 700 200 L = 10μH MAXIMUM INDUCTOR PEAK CURRENT (mA) 0 600 0 2 4 6 8 10 12 0 20 40 60 80 100 VIN (V) DUTY CYCLE (%) 1877 F01 1877 F02
Figure 1. Maximum Output Current vs Input Voltage Figure 2. Maximum Inductor Peak Current vs Duty Cycle APPLICATIONS INFORMATION
The basic LTC1877 application circuit is shown on the fi rst The operating frequency and inductor selection are inter- page. External component selection is driven by the load related in that higher operating frequencies allow the use of requirement and begins with the selection of L followed smaller inductor and capacitor values. However, operating by CIN and COUT. at a higher frequency generally results in lower effi ciency because of increased internal gate charge losses.
Inductor Value Calculation
The inductor value has a direct effect on ripple current. The inductor selection will depend on the operating fre- The ripple current ΔIL decreases with higher inductance quency of the LTC1877. The internal nominal frequency is or frequency and increases with higher VIN or VOUT. 550kHz, but can be externally synchronized from 400kHz 1 ⎛ V ⎞ OUT to 700kHz. ΔIL = V ⎜ ⎟ ( OUT 1− f ⎝ V ) L ( ) IN ⎠ (1) 1877fb 9