L6712

L6712 L6712A TWO-PHASE INTERLEAVED DC/DC CONTROLLER 1 ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Features 2 PHASE OPERATION WITH SYNCHRONOUS RECTIFIER CONTROL ULTRA FAST LOAD TRANSIENT RESPONSE INTEGRATED HIGH CURRENT GATE DRIVERS: UP TO 2A GATE CURRENT 3 BIT PROGRAMMABLE OUTPUT FROM 0.900V TO 3.300V OR WITH EXTERNAL REF. ±0.9% OUTPUT VOLTAGE ACCURACY 3mA CAPABLE AVAILABLE REFERENCE INTEGRATED PROGRAMMABLE REMOTE SENSE AMPLIFIER PROGRAMMABLE DROOP EFFECT 10% ACTIVE CURRENT SHARING ACCURACY DIGITAL 2048 STEP SOFT-START CROWBAR LATCHED OVERVOLTAGE PROT. NON-LATCHED UNDERVOLTAGE PROT. OVERCURRENT PROTECTION REALIZED USING THE LOWER MOSFET'S RdsON OR A SENSE RESISTOR OSCILLATOR EXTERNALLY ADJUSTABLE AND INTERNALLY FIXED AT 150kHZ POWER GOOD OUTPUT AND INHIBIT FUNCTION PACKAGES: SO-28 & VFQFPN-36 Figure 1. Packages SO28 VFQFPN-36 (6x6x1.0mm) Table 1. Order Codes Tube L6712D, SO L6712AD L6712Q, VFQFPN L6712AQ Package Tape & Reel L6712DTR, L6712ADTR L6712QTR, L6712AQTR ■ ■ ■ 1.1 Applications ■ HIGH CURRENT DC/DC CONVERTERS ■ DISTRIBUTED POWER SUPPLY 2 Description The device implements a dual-phase step-down controller with a 180 phase-shift between each phase optimized for high current DC/DC applications. Output voltage can be programmed through the integrated DAC from 0.900V to 3.300V; programming the "111" code, an external reference from 0.800V to 3.300V is used for the regulation. Programmable Remote Sense Amplifier avoids use of external resistor divider and recovers losses along distribution line. The device assures a fast protection against load over current and Over / Under voltage.An internal crowbar is provided turning on the low side mosfet if Over-voltage is detected. Output current is limited working in Constant Current mode: when Under Voltage is detected, the device resets, restarting operation. June 2005 Rev. 3 1/29 L6712A L6712 Figure 2. Block Diagram OSC / INH SGND VCCDR BOOT1 LOGIC PWM ADAPTIVE ANTI CROSS CONDUCTION BAND-GAP REFERENCE 2 PHASE OSCILLATOR HS UGATE1 PHASE1 PGOOD PWM1 CURRENT CORRECTION VID2 VID1 VID0 DAC LOGIC AND PROTECTIONS VCC VCCDR CH1 OCP LS LGATE1 ISEN1 CH1 OCP CH2 OCP TOTAL CURRENT CURRENT AVG CURRENT READING PGNDS1 PGND DIGITAL SOFT-START CURRENT READING PGNDS2 ISEN2 CURRENT CORRECTION REF_IN/OUT IDROOP CH2 OCP FBG IFB_START LOGIC PWM ADAPTIVE ANTI CROSS CONDUCTION VPROG LS LGATE2 PHASE2 HS UGATE2 BOOT2 FBR REMOTE AMPLIFIER PWM2 ERROR AMPLIFIER Vcc VSEN DROOP FB COMP Vcc Table 2. Absolute Maximum Ratings Symbol VCC, VCCDR VBOOT-VPHASE VUGATE1-VPHASE1 VUGATE2-VPHASE2 LGATE1, PHASE1, LGATE2, PHASE2 to PGND VID0 to VID2 All other pins to PGND VPHASEx UGATEX Pins OTHER PINS Sustainable Peak Voltage. T 35µA). ■ L6712 - Dynamic Maximum Duty Cycle Limitation The maximum duty cycle is limited as a function of the measured current and, since the oscillator frequency is fixed once programmed, imply a maximum on-time limitation as follow (where T is the switching period T=1/fSW and IOUT is the output current): ⎧ R SENSE ⎪ T = 0.80 ⋅ T I FB = 0 µ A T ON,MAX = ( 0.80 – I FB ⋅ 5.73k ) ⋅ T = ⎛ 0.80 – --------------------- ⋅ I OUT ⋅ 5.73k⎞ ⋅ T = ⎨ ⎝ ⎠ Rg ⎪ T = 0.40 ⋅ T I FB = 70 µ A ⎩ This linear dependence has a value at zero load of 0.80·T and at maximum current of 0.40·T typical and results in two different behaviors of the device: Figure 9. TON Limited Operation VOUT 0.80·VIN VOUT 0.80·VIN T ON Limited Output characteristic Resulting Output characteristic Desired Output characteristic and UVP threshold 0.40·VIN 0.40·VIN IOCP=2·IOCPx (IDROOP=70µA) IOUT IOCP=2·IOCPx (IDROOP=70µA) IOUT a) Maximum output Voltage b) TON Limited Output Voltage TON Limited Output Voltage. FOx This happens when the maximum ON time is reached before the current in each phase reaches IOCPx (IIN< 35µA). Figure 9a shows the maximum output voltage that the device is able to regulate considering the TON limitation imposed by the previous relationship. If the desired output characteristic crosses the TON limited maximum output voltage, the output resulting voltage will start to drop after crossing. In this case, the device doesn't perform constant current limitation but only limits the maximum duty cycle following the previous relationship. The output voltage follows the resulting characteristic (dotted in Figure 9b) until UVP is detected or anyway until IFB = 70µA. Constant Current Operation This happens when ON time limitation is reached after the current in each phase reaches IOCPx (IINFOx > 35µA). The device enters in Quasi-Constant-Current operation: the low-side mosfets stays ON until the current read becomes lower than IOCPx (IINFOx < 35µA) skipping clock cycles. The high side mosfets can be turned ON with a TON imposed by the control loop at the next available clock cycle and the device works in the 13/29 L6712A L6712 usual way until another OCP event is detected. This means that the average current delivered can slightly increase also in Over Current condition since the current ripple increases. In fact, the ON time increases due to the OFF time rise because of the current has to reach the IOCPx bottom. The worst-case condition is when the ON time reaches its maximum value. When this happens, the device works in Constant Current and the output voltage decrease as the load increase. Crossing the UVP threshold causes the device to reset. Figure 10 shows this working condition. It can be observed that the peak current (Ipeak) is greater than the IOCPx but it can be determined as follow: V IN – Vout min V IN – Vout MIN I peak = I OCPx + ------------------------------------- ⋅ Ton MAX = I OCPx + -------------------------------------- ⋅ 0.40 ⋅ T L L Where VoutMIN is the minimum output voltage (VID-40% as follow). The device works in Constant-Current, and the output voltage decreases as the load increase, until the output voltage reaches the under-voltage threshold (VoutMIN). The maximum average current during the Constant-Current behavior results: Ipeak – I OCPx I MAX,TOT = 2 ⋅ I MAX = 2 ⋅ ⎛ I OCPx + ------------------------------------- ⎞ ⎝ ⎠ 2 In this particular situation, the switching frequency results reduced. The ON time is the maximum allowed (TonMAX) while the OFF time depends on the application: Ipeak – I OCPx T OFF = L ⋅ ------------------------------------V OUt Figure 10. Constant Current operation Ipeak IMAX IOCPx UVP Vout Droop effect 1 f = ----------------------------------------T ONmax + T OFF TonMAX TonMAX (IDROOP=50µA) Iout IMAX,TOT 2·IOCPx (IDROOP=70µA) a) Maximum current for each phase b) Output Characteristic Over current is set anyway when IINFOx reaches 35µA (IFB=70µA). The full load value is only a convention to work with convenient values for IFB. Since the OCP intervention threshold is fixed, to modify the percentage with respect to the load value, it can be simply considered that, for example, to have on OCP threshold of 200%, this will correspond to IINFOx = 35µA (IFB = 70µA). The full load current will then correspond to IINFOx = 17.5µA (IFB = 35µA). Once the UVP threshold has been intercepted, the device resets with all power mosfets turned OFF. Another soft start is then performed allowing the device to recover from OCP once the over load cause has been removed. Crossing the UVP threshold causes the device to reset: all mosfets are turned off and a new soft start is 14/29 L6712A L6712 then implemented allowing the device to recover if the over load cause has been removed. ■ L6712A - Fixed Maximum Duty Cycle Limitation The maximum duty cycle is fixed and constant with the delivered current. The device works in constant current operation once the OCP threshold has overcome. Refer to the above Constant Current section in which only the different value in the maximum duty has to be considered as follow: V IN – Vout min V IN – Vout MIN I peak = I OCPx + ------------------------------------- ⋅ Ton MAX = I OCPx + -------------------------------------- ⋅ 0.85 ⋅ T L L All the above reported relationships about the deliverable current once in quasi-constant current and constant current are still valid in this case. 3.5 REMOTE SENSE AMPLIFIER Remote Sense Amplifier is integrated in order to recover from losses across PCB traces and wiring in high current DC/DC converter remote sense of the regulated voltage is required to maintain precision in the regulation. The integrated amplifier is a low-offset error amplifier; external resistors are needed as shown in Figure 11 to implement a differential remote sense amplifier. Figure 11. Remote Sense Amplifier Connections Reference Reference ERROR AMPLIFIER REMOTE AMPLIFIER ERROR AMPLIFIER IDROOP REMOTE AMPLIFIER IDROOP FBR FBG VSEN DROOP FB COMP FBR FBG VSEN DROOP FB COMP R2 R1 Remote VOUT R2 R1 Remote Ground RFB CF RF RFB CF RF VOUT RB used RB Not Used Equal resistors give to the resulting amplifier a unity gain: the programmed reference will be regulated across the remote load. To regulate output voltages different from the available references, the Remote Amplifier gain can be adjusted simply changing the value of the external resistors as follow (see Figure 11): V VSEN RA_Gain = ---------------------------------------------------------------------------------------- = R2 ------Remote_V OUT – Remote_GND R1 to regulate a voltage double of the reference, the above reported gain must be equal to ½. Modifying the Remote Amplifier Gain (in particular with values higher than 1) allows also to regulate voltages lower than the programmed reference. Since this Amplifier is connected as a differential amplifier, when calculating the offset introduced in the regulated output voltage, the "native" offset of the amplifier must be multiplied by the term KOS = [1+(1/RA_Gain)] because a voltage generator insisting on the non-inverting input represents the offset. If remote sense is not required, it is enough connecting RFB directly to the regulated voltage: VSEN becomes not connected and still senses the output voltage through the remote amplifier. In this case the use of the external resistors R1 and R2 becomes optional and the Remote Sense Amplifier can simply be connected as a "buffer" to keep VSEN at the regulated voltage (See Figure 11). Avoiding use of Remote Amplifier saves its offset in the accuracy calculation but doesn't allow remote sensing. 15/29 L6712A L6712 3.6 INTEGRATED DROOP FUNCTION (Optional) Droop function realizes dependence between the regulated voltage and the delivered current (Load Regulation). In this way, a part of the drop due to the output capacitor ESR in the load transient is recovered. As shown in Figure 12, the ESR drop is present in any case, but using the droop function the total deviation of the output voltage is minimized. Connecting DROOP pin and FB pin together, forces a current IDROOP, proportional to the output current, into the feedback resistor RFB implementing the load regulation dependence. If RA_Gain is the Remote Amplifier gain, the Output Characteristic is then given by the following relationship (when droop enabled): R SENSE 1 1 V OUT = ------------------------ ⋅ ( VID – R FB ⋅ I DROOP ) = ------------------------ ⋅ ⎛ VID – R FB ⋅ --------------------- ⋅ I OUT⎞ ⎠ RA_Gain ⎝ RA_Gain Rg with a remote amplifier gain of 1/2, the regulated output voltage results in being doubled. The Droop current is equal to 50µA at nominal full load and 70µA at the OC intervention threshold, so the maximum output voltage deviation is equal to: 1 ∆ V FULL – POSITIVE – LOAD = – ------------------------ ⋅ R ⋅ 50 µ A RA_Gain FB 1 ∆ V OC – INTERVENTION = – ------------------------ ⋅ R ⋅ 70 µ A RA_Gain FB Droop function is provided only for positive load; if negative load is applied, and then IINFOx