Datasheet LT8708-1 (Analog Devices) - 10

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PIN FUNCTIONS CLKOUT (Pin 1):
Clock Output Pin. Use this pin to syn-
FBOUT (Pin 9):
VOUT Feedback Pin. This pin is connected chronize one or more compatible switching regulator ICs. to the input of error amplifier EA4. Typically, connect this CLKOUT toggles at the same frequency as the internal pin to GND to disable the EA4. oscillator or as the SYNC pin, but is approximately 180°
V
out of phase. CLKOUT may also be used as a temperature
C (Pin 10):
Error Amplifier Output Pin. Tie external com- pensation network to this pin. monitor since the CLKOUT duty cycle varies linearly with the part’s junction temperature. The CLKOUT pin can drive
IMON_INP (Pin 11):
Positive VIN Current Monitor and capacitive loads up to 200pF. Limit Pin. The current out of this pin is 20µA plus a cur- rent proportional to the positive average V
SS (Pin 2):
Soft-Start Pin. Place a capacitor from this pin IN current. IMON_INP also connects to error amplifier EA5 and can to ground. A capacitor identical to the SS pin capacitor be used to limit the maximum positive V used on the master LT8708 is recommended. Upon start- IN current. See the Applications Information section for more information. up, this pin will be charged by an internal resistor to 3.3V.
IMON_INN (Pin 12):
Negative V
SHDN (Pin 3):
Shutdown Pin. Tie high to enable chip. IN Current Monitor and Limit Pin. The current out of this pin is 20µA plus a cur- Ground to shut down and reduce quiescent current to a rent proportional to the negative average V minimum. Do not float this pin. IN current. IMON_INN also connects to error amplifier EA1 and can
CSN (Pin 4):
The (–) Input to the Inductor Current Sense be used to limit the maximum negative VIN current. See and DCM Detect Comparator. the Applications Information section for more information.
CSP (Pin 5):
The (+) Input to the Inductor Current Sense
RT (Pin 13):
Timing Resistor Pin. Adjusts the switching and DCM Detect Comparator. The VC pin voltage and built- frequency. Place a resistor from this pin to ground to set in offsets between CSP and CSN pins, in conjunction with the frequency. It is recommended to use the same value the RSENSE value, set the inductor current trip threshold. RT resistor as the master LT8708. Do not float this pin. It is recommended to use the same value RSENSE as the
SYNC (Pin 14):
To synchronize the switching frequency master LT8708. to an outside clock, simply drive this pin with a clock. The
ICN (Pin 6):
Negative VOUT Current Command Pin. The high voltage level of the clock needs to exceed 1.3V, and voltage on this pin determines the negative VOUT current the low level should be less than 0.5V. In a two-phase for LT8708-1 to regulate to. Connect this pin to the master system, connect this pin to the master LT8708’s CLKOUT LT8708’s ICN pin. See the Applications Information sec- pin to have a 180° phase shift. See the Applications tion for more information. Information section for more information.
DIR (Pin 7):
Direction pin when MODE is set for DCM
BG1, BG2 (Pin 16, Pin 18):
Bottom Gate Drive. Drives the (discontinuous conduction mode) or HCM (hybrid con- gate of the bottom N-channel MOSFETs between ground duction mode) operation. Otherwise this pin is ignored. and GATEVCC. Connect the pin to GND to process power from the VOUT
GATEV
to V
CC (Pin 17):
Power supply for bottom gate drivers. IN. Connect the pin to LDO33 to process power from Must be connected to the INTV the V CC pin. Do not power from IN to VOUT. Drive this pin with the same control any other supply. Locally bypass to GND. It is recom- signal, or connect this pin to the same voltages as the mended to use the same value bypass cap as the master master LT8708. LT8708.
FBIN (Pin 8):
VIN Feedback Pin. This pin is connected to the input of error amplifier EA3. Typically, connect this pin to LDO33 to disable the EA3. Rev 0 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Common LT8708-1 and LT8708 Features Adding Phases to an LT8708 Application Adding Phases: The Master LT8708 Adding Phases: The Slave LT8708-1 Start-Up Start-Up: SWEN Pin Start-Up: Soft-Start of Switching Regulator Control Overview Power Switch Control Unidirectional and Bidirectional Conduction Error Amplifiers Transfer Function: IOUT(SLAVE) Vs IOUT(MASTER) Transfer Function: CCM Transfer Function: DCM, HCM and Burst Mode Operation Current Monitoring and Limiting Monitoring: IOUT(SLAVE) Monitoring and Limiting: IIN(SLAVE) Multiphase Clocking Applications Information Quick-Start Multiphase Setup Quick Setup: Design the Master Phase Quick Setup: Design the Slave Phase(s) Quick Setup: Evaluation Choosing the Total Number of Phases Operating Frequency Selection CIN and COUT Selection CIN and COUT Selection: VIN Capacitance CIN and COUT Selection: VOUT Capacitance VINHIMON, VOUTLOMON and RVSOFF Configuring the IIN(SLAVE) Current Limits Regulating IOUT(SLAVE) IOUT(SLAVE): Circuit Description IOUT(SLAVE): Configuration Loop Compensation Voltage Lockouts Circuit Board Layout Checklist Design Example Typical Applications Package Description Typical Application Related Parts