Datasheet ADP2105, ADP2106, ADP2107 (Analog Devices) - 15

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Data Sheet ADP2105/ADP2106/ADP2107 THEORY OF OPERATION
The ADP2105/ADP2106/ADP2107 are step-down, dc-to-dc
PFM MODE OPERATION
converters that use a fixed frequency, peak current mode archi- The ADP2105/ADP2106/ADP2107 smoothly transition to the tecture with an integrated high-side switch and low-side synchron- variable frequency PFM mode of operation when the load current ous rectifier. The high 1.2 MHz switching frequency and tiny decreases below the pulse skipping threshold current, switching 16-lead, 4 mm × 4 mm LFCSP package allow for a small step- only as necessary to maintain the output voltage within regulation. down dc-to-dc converter solution. The integrated high-side switch When the output voltage dips below regulation, the ADP2105/ (P-channel MOSFET) and synchronous rectifier (N-channel ADP2106/ADP2107 enter PWM mode for a few oscil ator cycles MOSFET) yield high efficiency at medium to heavy loads. Light to increase the output voltage back to regulation. During the wait load efficiency is improved by smoothly transitioning to variable time between bursts, both power switches are off, and the output frequency PFM mode. capacitor supplies all the load current. Because the output voltage The ADP2105/ADP2106/ADP2107 (ADJ) operate with an input dips and recovers occasionally, the output voltage ripple in this voltage from 2.7 V to 5.5 V and regulate an output voltage down to mode is larger than the ripple in the PWM mode of operation. 0.8 V. The ADP2105/ADP2106/ADP2107 are also available with
PULSE-SKIPPING THRESHOLD
preset output voltage options of 3.3 V, 1.8 V, 1.5 V, and 1.2 V. The output current at which the ADP2105/ADP2106/ADP2107
CONTROL SCHEME
transition from variable frequency PFM control to fixed frequency The ADP2105/ADP2106/ADP2107 operate with a fixed PWM control is called the pulse-skipping threshold. The pulse- frequency, peak current mode PWM control architecture at skipping threshold is optimized for excellent efficiency over al medium to high loads for high efficiency, but shift to a variable load currents. The variation of pulse-skipping threshold with frequency PFM control scheme at light loads for lower quies- input voltage and output voltage is shown in Figure 25, Figure 27, cent current. When operating in fixed frequency PWM mode, and Figure 28. the duty cycle of the integrated switches is adjusted to regulate
100% DUTY CYCLE OPERATION (LDO MODE)
the output voltage, but when operating in PFM mode at light loads, the switching frequency is adjusted to regulate the output As the input voltage drops, approaching the output voltage, the voltage. ADP2105/ADP2106/ADP2107 smoothly transition to 100% duty cycle, maintaining the P-channel MOSFET switch-on continuously. The ADP2105/ADP2106/ADP2107 operate in the PWM mode This allows the ADP2105/ADP2106/ADP2107 to regulate the only when the load current is greater than the pulse-skipping output voltage until the drop in input voltage forces the P-channel threshold current. At load currents below this value, the converter MOSFET switch to enter dropout, as shown in the fol owing smoothly transitions to the PFM mode of operation. equation:
PWM MODE OPERATION
VIN(MIN) = IOUT × (RDS(ON) − P + DCRIND) + VOUT(NOM) In PWM mode, the ADP2105/ADP2106/ADP2107 operate at a The ADP2105/ADP2106/ADP2107 achieve 100% duty cycle fixed frequency of 1.2 MHz set by an internal oscillator. At the operation by stretching the P-channel MOSFET switch-on time start of each oscillator cycle, the P-channel MOSFET switch is if the inductor current does not reach the peak inductor current turned on, putting a positive voltage across the inductor. Current level by the end of the clock cycle. When this happens, the oscil- in the inductor increases until the current sense signal crosses lator remains off until the inductor current reaches the peak the peak inductor current level that turns off the P-channel inductor current level, at which time the switch is turned off and MOSFET switch and turns on the N-channel MOSFET synchro- the synchronous rectifier is turned on for a fixed off time. At nous rectifier. This puts a negative voltage across the inductor, the end of the fixed off time, another cycle is initiated. As the causing the inductor current to decrease. The synchronous ADP2105/ADP2106/ADP2107 approach dropout, the switching rectifier stays on for the remainder of the cycle, unless the frequency decreases gradually to smoothly transition to 100% inductor current reaches zero, which causes the zero-crossing duty cycle operation. comparator to turn off the N-channel MOSFET. The peak inductor current is set by the voltage on the COMP pin. The COMP pin is the output of a transconductance error amplifier that compares the feedback voltage with an internal 0.8 V reference. Rev. E | Page 15 of 36 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION TYPICAL OPERATING CIRCUIT REVISION HISTORY FUNCTIONAL BLOCK DIAGRAM SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE BOUNDARY CONDITION ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION CONTROL SCHEME PWM MODE OPERATION PFM MODE OPERATION PULSE-SKIPPING THRESHOLD 100% DUTY CYCLE OPERATION (LDO MODE) DESIGN FEATURES Enable/Shutdown Synchronous Rectification Current Limit Short-Circuit Protection Undervoltage Lockout (UVLO) Thermal Protection Soft Start APPLICATIONS INFORMATION ADIsimPower DESIGN TOOL EXTERNAL COMPONENT SELECTION SETTING THE OUTPUT VOLTAGE INDUCTOR SELECTION OUTPUT CAPACITOR SELECTION INPUT CAPACITOR SELECTION INPUT FILTER SOFT START PERIOD LOOP COMPENSATION BODE PLOTS LOAD TRANSIENT RESPONSE EFFICIENCY CONSIDERATIONS Power Switch Conduction Losses Inductor Losses Switching Losses Transition Losses THERMAL CONSIDERATIONS DESIGN EXAMPLE EXTERNAL COMPONENT RECOMMENDATIONS CIRCUIT BOARD LAYOUT RECOMMENDATIONS EVALUATION BOARD EVALUATION BOARD SCHEMATIC FOR ADP2107 (1.8 V) RECOMMENDED PCB LAYOUT (EVALUATION BOARD LAYOUT) APPLICATION CIRCUITS OUTLINE DIMENSIONS ORDERING GUIDE