Datasheet LTC1878 (Analog Devices) - 8

LTC1878
U OPERATIO Dropout Operation
Another important detail to remember is that at low input supply voltages, the R When the input supply voltage decreases toward the DS(ON) of the P-channel switch increases. Therefore, the user should calculate the power output voltage, the duty cycle increases toward the maxi- dissipation when the LTC1878 is used at 100% duty cycle mum on-time. Further reduction of the supply voltage with a low input voltage (see Thermal Considerations in forces the main switch to remain on for more than one the Applications Information section). cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the
Slope Compensation and Inductor Peak Current
voltage drop across the internal P-channel MOSFET and the inductor. Slope compensation provides stability in constant fre- quency architectures by preventing subharmonic oscilla-
Low Supply Operation
tions at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current The LTC1878 is designed to operate down to an input signal at duty cycles in excess of 40%. As a result, the supply voltage of 2.65V although the maximum allowable maximum inductor peak current is reduced for duty cycles output current is reduced at this low voltage. Figure 1 > 40%. This is shown in the decrease of the inductor peak shows the reduction in the maximum output current as a current as a function of duty cycle graph in Figure 2. function of input voltage for various output voltages. 1200 1100 L = 10µH VIN = 3.3V 1000 1000 VOUT = 1.5V 800 VOUT = 3.3V 900 600 VOUT = 2.5V 800 400 MAX OUTPUT CURRENT (mA) 700 200 MAXIMUM INDUCTOR PEAK CURRENT (mA) 0 600 2.5 3.5 4.5 5.5 6.5 7.5 0 20 40 60 80 100 INPUT VOLTAGE (V) DUTY CYCLE (%) 1878 F01 1878 F02
Figure 1. Maximum Output Current vs Input Voltage Figure 2. Maximum Inductor Peak Current vs Duty Cycle U U W U APPLICATIO S I FOR ATIO
The basic LTC1878 application circuit is shown on the first 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 requirement and begins with the selection of L followed by of smaller inductor and capacitor values. However, oper- CIN and COUT. ating at a higher frequency generally results in lower efficiency because of increased internal gate charge losses.
Inductor Value Calculation
The inductor value has a direct effect on ripple current. The The inductor selection will depend on the operating fre- ripple current ∆IL decreases with higher inductance or quency of the LTC1878. The internal nominal frequency is frequency and increases with higher VIN or VOUT. 550kHz, but can be externally synchronized from 400kHz to 700kHz. 8