Datasheet LTM4691 (Analog Devices) - 10

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APPLICATIONS INFORMATION
The typical LTM4691 application circuit is shown on V page 1. External component selection is primarily deter- OUT + RTOP CFF C mined by the input voltage, the output voltage and the OUT BUCK SWITCHING FB maximum load current. Refer to Table 4 for specific exter- REGULATOR RBOT nal capacitor requirements for a particular application.
V
4691 F01
IN to VOUT Step-Down Ratios Figure 1. Feedback Components
There are restrictions in the minimum VOUT step-down ratio that can be achieved for a given input voltage due to
Input Decoupling Capacitors
the minimum on-time limits of the regulator. The LTM4691 module should be connected to a low The minimum on-time limit imposes a minimum duty AC-impedance DC source. For the regulator, one-piece cycle of the converter which can be calculated using 10µF input ceramic capacitor near each VIN pin is recom- Equation 1. mended for RMS ripple current decoupling. Bulk input (1) capacitor is only needed when the input source imped- D MIN = tON(MIN) • fSW ance is compromised by long inductive leads, traces or where t not enough source capacitance. The bulk capacitor can be ON(MIN) is the minimum on-time, 35ns typical for LTM4691. In rare cases where the minimum duty cycle an electrolytic aluminum capacitor and polymer capacitor. is surpassed, the output voltage will remain in regula- Without considering the inductor current ripple, the RMS tion, but the switching frequency will decrease from its current of the input capacitor can be estimated as shown programmed value. in Equation 3. There is no maximum VOUT step-down ratio limitation for IOUT(MAX) (3) the LTM4691. Operating at 100% duty-cycle low dropout, ICIN(RMS) = • D • (1– D) η% the output voltage of the LTM4691 could be as high as 2.5V. where η% is the estimated efficiency of the power module.
Output Voltage Programming Output Decoupling Capacitors
The PWM controller has an internal 0.5V reference volt- With an optimized high frequency, high bandwidth age. Adding a resistor divider from VOUT remote sensing design, only one 22μF low ESR output ceramic capacitor point to FB pin and from FB pin to SGND pin programs is required for LTM4691 to achieve low output voltage the output voltage as shown in Equation 2. ripple and very good transient response. Additional output R filtering may be required by the system designer, if further V TOP + RBOT OUT = 0.5V • (2) reduction of output ripples or dynamic transient spikes is RBOT required. Table 4 shows a matrix of different output volt- 1% resistors are recommended to maintain output volt- ages and output capacitors to minimize the voltage droop age accuracy. The buck regulator transient response may and overshoot during a 500mA (25%) load step transient. improve with an optional phase lead capacitor CFF that helps cancel the pole created by the feedback resistors
Mode Selection
and the input capacitance of the FB pin (Figure 1). The buck switching regulators can operate in three differ- ent modes by setting the MODE/SYNC pin: pulse-skipping mode (when the MODE/SYNC pin is set low), forced con- tinuous PWM mode (when the MODE/SYNC pin is float- ing), and Burst Mode operation (when the MODE/SYNC Rev. 0 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information VIN to VOUT Step-Down Ratios Output Voltage Programming Input Decoupling Capacitors Output Decoupling Capacitors Mode Selection Operating Frequency and External Synchronization Power GOOD Output Overvoltage Protection Output Voltage Soft-Start Dropout Operation Output Short-Circuit Protection and Recovery Load Sharing Using the Precision RUN Threshold Thermal Considerations and Output Current Derating Safety Considerations Layout Checklist/Example Applications Information Typical Applications Pin Configuration Table LTM4691 Component LGA Pinout Package Description Package Photo Related Parts