ISL62871EVAL2Z Rev A Evaluation Board User Guide APPLICATION NOTE AN1441 Rev 0.00 Page 1 of 16 Nov 17, 2008 AN1441 Rev 0.00 Nov 17, 2008 Introduction The ISL62870EVAL2Z Rev A evaluation board demonstrates the performance of the ISL62870HRUZ single-phase synchronous-buck PWM controller featuring Intersil's Robust Ripple Regulator (R 3 ) technology. An on-board dynamic-load generator is included for evaluating the transient-load response, which applies a pulse of 125macross V OUT and GND. Contents of this document include: • Design Criteria • Component Selection • Recommended Test Equipment • Interface Connections • Switch Descriptions • Jumper Descriptions • Test Point Descriptions • Typical Load-transient Response Waveforms • Evaluation Board Documentation - Schematic - Silk-screen plots - Board layer plots - Bill of materials Design Criteria Component Selection Output Setpoint Voltage Programming The procedure for programming the output setpoint voltage requires only the component selection for the feedback divider network that attenuates the output voltage and applies it to the FB pin. The voltage at the FB pin is regulated to equal V SREF which is equal to the 500mV internal reference V REF once the soft-start sequence has completed. CALCULATE THE ATTENUATION CONSTANT “K” Calculate K with Equation 1: CALCULATE R OFS USING ATTENUATION CONSTANT K The feedback voltage divider network consists of two resistors R FB and R OFS where: - R FB is the loop-compensation feedback resistor that connects from the FB pin of the IC to the positive voltage of the converter output - R OFS is the scaling resistor that connects from the FB pin of the IC to GND, which must be recalculated after each instance of changing the value of resistor R FB . Calculate R OFS with Equation 2: The results from Equation 2 must be rounded to the next highest or next lowest standard resistor value that most accurately reproduces K using Equation 3: Check V OUT output setpoint voltage with Equation 4: SOFT-START DELAY PROGRAMMING Calculate Value for C SOFT The elapsed time from when the EN pin is asserted to when V SREF has charged C SOFT to V REF is called the soft-start delay t SS which is given by Equation 5: Where: - I SS is the soft-start current source at the 20µA limit - V SREF is 500mV The end of soft-start is detected by I SS tapering off when capacitor C SOFT charges to V SREF . The internal SSOK flag TABLE 1. DC/DC DESIGN CRITERIA PARAMETER VALUE UNITS V IN 3.3V to 25V VDC V OUT 1.20 VDC MAX DC LOAD 20 ADC t SS 1.5 ms OCP THRESHOLD 30 ADC K V SREF V OUT ------------------ = (EQ. 1) R OFS K R FB 1 K – ------------------- = (EQ. 2) K R OFS R FB R OFS + --------------------------------- = (EQ. 3) V OUT V SREF K ------------------ = (EQ. 4) t SS V SREF C SOFT I SS ------------------------------------------- = (EQ. 5)
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ISL62871EVAL2Z Rev A Evaluation Board User Guide
APPLICATION NOTE
AN1441Rev 0.00
Nov 17, 2008
IntroductionThe ISL62870EVAL2Z Rev A evaluation board demonstrates the performance of the ISL62870HRUZ single-phase synchronous-buck PWM controller featuring Intersil's Robust Ripple Regulator (R3) technology. An on-board dynamic-load generator is included for evaluating the transient-load response, which applies a pulse of 125m across VOUT and GND. Contents of this document include:
• Design Criteria
• Component Selection
• Recommended Test Equipment
• Interface Connections
• Switch Descriptions
• Jumper Descriptions
• Test Point Descriptions
• Typical Load-transient Response Waveforms
• Evaluation Board Documentation
- Schematic
- Silk-screen plots
- Board layer plots
- Bill of materials
Design Criteria
Component Selection
Output Setpoint Voltage Programming
The procedure for programming the output setpoint voltage requires only the component selection for the feedback divider network that attenuates the output voltage and applies it to the FB pin. The voltage at the FB pin is regulated to equal VSREF which is equal to the 500mV internal reference VREF once the soft-start sequence has completed.
CALCULATE THE ATTENUATION CONSTANT “K”
Calculate K with Equation 1:
CALCULATE ROFS USING ATTENUATION CONSTANT K
The feedback voltage divider network consists of two resistors RFB and ROFS where:
- RFB is the loop-compensation feedback resistor that connects from the FB pin of the IC to the positive voltage of the converter output
- ROFS is the scaling resistor that connects from the FB pin of the IC to GND, which must be recalculated after each instance of changing the value of resistor RFB.
Calculate ROFS with Equation 2:
The results from Equation 2 must be rounded to the next highest or next lowest standard resistor value that most accurately reproduces K using Equation 3:
Check VOUT output setpoint voltage with Equation 4:
SOFT-START DELAY PROGRAMMING
Calculate Value for CSOFT
The elapsed time from when the EN pin is asserted to when VSREF has charged CSOFT to VREF is called the soft-start delay tSS which is given by Equation 5:
Where:
- ISS is the soft-start current source at the 20µA limit
- VSREF is 500mV
The end of soft-start is detected by ISS tapering off when capacitor CSOFT charges to VSREF. The internal SSOK flag
is set, the PGOOD pin goes high, and diode emulation mode (DEM) is enabled.
Component Selection For CSOFT Capacitor
Choosing the CSOFT capacitor to meet the requirements of a particular soft-start delay tSS is calculated using Equation 6, written as follows:
Where:
- tSS is the soft-start delay
- ISS is the 20µA soft-start current source at the 20µA limit
- VSREF is 500mV
DCR-SENSE OVERCURRENT THRESHOLD PROGRAMMING
DCR-sensing for overcurrent protection OCP, is accomplished by detecting the voltage created across the DCR of the output inductor LO.
VDCR is calculated with Equation 7:
A series-connected RC network consisting of resistor ROCSET and capacitor CSEN are connected across the centers of the PCB pads where the output inductor LO is mounted. Resistor ROCSET connects to the phase side of LO while capacitor CSEN connects to the VOUT side of LO.
The OCSET pin sinks the 10µA programming current reference IOCSET continuously through resistor ROCSET creating a voltage VROCSET that is equal to VDCR at the specified overcurrent threshold through ILO.
VROCSET is calculated with Equation 8:
The VO pin is connected through resistor RO to the VOUT side of capacitor CSEN and is the reference to which the OCSET pin is compared. The value of RO must always match the value of ROCSET.
COMPONENT SELECTION FOR ROCSET AND CSEN
Calculate ROCSET with Equation 9:
Where:
- ROCSET is the resistor used to program the overcurrent setpoint
- IOC is the output DC load current that will activate the OCP fault detection circuit
- DCR is the inductor DC resistance
- IOCSET is the 10µA programming current reference that is sinked into the OCSET pin
To sense the inductor current correctly during dynamic operation, the RC time constant ROCSET •CSEN needs to match LO/DCR.
Calculate CSEN with Equation 10:
Calculations for ISL62870EVAL2Z Rev A Component Values
CALCULATE THE ATTENUATION CONSTANT "K"
Using the criteria from Table 1.
- VOUT is 1.2V
From Equation 1, K is:
The value to be used for RIN shall be the typical value of 2.49k.
From Equation 2, ROFS is:
SELECT NEAREST STANDARD RESISTOR VALUE FOR THE CALCULATED ROFS COMPONENT VALUE AND CHECK RESULTANT OUTPUT SETPOINT VOLTAGE:
From Equation 3 K is:
From Equation 4 VOUT is:
CALCULATE THE VALUE FOR CSOFT AS A FUNCTION OF THE tSS REQUIREMENT
For this application, the soft-start delay needs to complete at approximately 1.5ms:
SELECT NEAREST STANDARD CAPACITOR VALUE FOR CSOFT AND CHECK RESULTANT tSS
The closest standard capacitor value for the calculated value of CSOFT:
From Equation 5 tSS is:
DCR-SENSE OVERCURRENT THRESHOLD PROGRAMMING
From Equation 9 ROCSET is:
Select the nearest standard resistor value for ROCSET:
From Equation 10 CSEN is:
Select the nearest standard capacitor value for CSEN:
Recommended Equipment• (QTY 1) Adjustable 25V, 10A Power Supply
• (QTY 1) Fixed 12V, 100mA Power Supply
• (QTY 1) Fixed 5V, 100mA Power Supply
• (QTY 1) Adjustable 20A Constant Current Electronic Load
• (QTY 1) DVM
• (QTY 1) 4-Channel Oscilloscope
Interface Connections• VIN: Input voltage to the power stage of the converter
- J1: VIN positive power input
- J2: VIN return power input
• VOUT: Regulated output voltage from the converter
- J19: VOUT positive power output
- J17: VOUT return power output
• +5VIN: +5V input voltage for PVCC, PGOOD-LED, and pull-up voltage rail
- J9: 5V positive input
- J12: 5V return input
• +12V: Input voltage for the dynamic-load generator
- J14: 12V positive input
- J13: 12V return input
Switch Descriptions• S1: ENABLE
- OFF: Shorts the EN pin to GND (disable PWM)
- ON: Allows the EN pin to pull-up to +5V (enable PWM)
• S4: DYNAMIC LOAD
- ON: Load enabled
- OFF: Load disabled
Jumper Descriptions• J3: Connects +5V supply to the PGOOD LED circuit. The
shunt jumper is normally installed and should be removed when making low power efficiency measurements.
• J21: Isolates the EN input pin from switch S1 so that an external signal can be used. The shunt jumper is normally installed.
• J20: Isolates the PGOOD input pin from the pull-up resistor so that the pull-down resistance can be measured. The shunt jumper is normally installed and should be removed when making low power efficiency measurements.
• J6: Selects DCR current sense signal source. Short across PCB pads with a solder bridge to select.
• J7: Selects shunt-resistor current sense signal source. Short across PCB pads with a solder bridge to select.
• J10: Selects shunt-resistor current sense signal source. Short across PCB pads with a solder bridge to select.
• J11: Selects DCR current sense signal source. Short across PCB pads with a solder bridge to select.
Test-point Descriptions• TP3: EN pin
• TP5: PGOOD pin
• TP18: VIN positive voltage sense
• TP15: VIN return voltage sense
• TP32: VOUT positive voltage sense
• TP14: VOUT return voltage sense
• J15: Scope-probe socket for measuring the PHASE node
• J16: Scope-probe socket for measuring VOUT
• J18: Scope-probe socket for measuring voltage across the dynamic-load resistors (hence load current)
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