LM34927 Integrated Secondary Side Bias Regulator for Isolated DCDC Converters General Description Features The LM34927 regulator features all of the functions needed to implement a low cost, efficient, isolated bias regulator. This high voltage regulator contains two 100V N-Channel MOSFET switches - a high-side buck switch and a low-side synchronous switch. The Constant-on-time (COT) control scheme employed in the LM34927 requires no loop compensation and provides excellent transient response. The regulator operates with an on-time that is inversely proportional to the input voltage. This feature allows the operating frequency to remain relatively constant. An intelligent peak current limit is implemented with integrated sense circuit. Other features include a programmable input under voltage comparator to inhibit operation during low-voltage conditions. Protection features include thermal shutdown and VCC Undervoltage Lockout (UVLO). The LM34927 is offered in LLP-8 and PSOP-8 plastic packages. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide 9V to 100V Input Range Integrated 100V, High and Low Side Switches No Schottky Required Constant On-Time Control No Loop Compensation Required Ultra-Fast Transient Response Nearly Constant Operating Frequency Intelligent Peak Current Limit Adjustable Output Voltage From 1.225V Precision 2% Feedback Reference Frequency Adjustable to 1MHz Adjustable Undervoltage Lockout (UVLO) Remote Shutdown Thermal Shutdown Packages ■ LLP-8 ■ PSOP-8 Applications ■ Isolated Telecom Bias Supply ■ Isolated Automotive and Industrial Electronics Typical Application 30177901 FIGURE 1. Typical Application Schematic © 2012 Texas Instruments Incorporated 301779 SNVS799 www.ti.com LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters April 5, 2012 LM34927 Connection Diagram 30177903 Top View (Connect Exposed Pad to RTN) 30177902 Top View (Connect Exposed Pad to RTN) Ordering Information Order Number Package Type Package Drawing Supplied As LM34927MR PSOP-8 MRA08A 1000 Units on Tape and Reel LM34927SD LLP-8 SDC08B 1000 Units on Tape and Reel Pin Descriptions Pin Name 1 RTN 2 VIN 3 UVLO 4 Description Application Information Ground Ground connection of the integrated circuit. Input Voltage Operating input range is 9V to 100V. Input Pin of Undervoltage Comparator Resistor divider from VIN to UVLO to GND programs the undervoltage detection threshold. An internal current source is enabled when UVLO is above 1.225V to provide hysteresis. When UVLO pin is pulled below 0.66V externally, the parts goes in shutdown mode. RON On-Time Control A resistor between this pin and VIN sets the switch ontime as a function of VIN. Minimum recommended ontime is 100ns at max input voltage. 5 FB Feedback This pin is connected to the inverting input of the internal regulation comparator. The regulation level is 1.225V. 6 VCC Output from the Internal High Voltage Series Pass The internal VCC regulator provides bias supply for the Regulator. Regulated at 7.6V. gate drivers and other internal circuitry. A 1.0μF decoupling capacitor is recommended. 7 BST Bootstrap Capacitor An external capacitor is required between the BST and SW pins (0.01μF ceramic). The BST pin capacitor is charged by the VCC regulator through an internal diode when the SW pin is low. 8 SW Switching Node Power switching node. Connect to the output inductor and bootstrap capacitor. EP Exposed Pad Exposed pad must be connected to RTN pin. Connect to system ground plane on application board for reduced thermal resistance. www.ti.com 2 If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. VIN, UVLO to RTN SW to RTN BST to VCC BST to SW RON to RTN VCC to RTN -0.3V to 100V -1.5V to VIN +0.3V 100V 13V -0.3V to 100V -0.3V to 13V Operating Ratings -0.3V to 5V 2kV 200°C -55°C to +150°C (Note 1) VIN Voltage Operating Junction Temperature 9V to 100V −40°C to +125°C Electrical Characteristics Specifications with standard typeface are for TJ = 25°C, and those with boldface type apply over full Operating Junction Temperature range. VIN = 48V, unless otherwise stated. See (Note 3). Symbol Parameter Conditions Min Typ Max Units 6.25 7.6 8.55 V 4.5 4.9 VCC Supply VCC Reg VCC Regulator Output VIN = 48V, ICC = 20mA VCC Current Limit VIN = 48V(Note 4) VCC UVLO Threshold (VCC increasing) mA 26 4.15 VCC UVLO Hysteresis V 300 mV VCC Drop Out Voltage VIN = 8V, ICC = 20mA 2.3 V IIN Operating Current Non-Switching, FB = 3V 1.75 IIN Shutdown Current UVLO = 0V mA 50 225 µA Under-Voltage Sensing Function UV Threshold UV Rising 1.19 1.225 1.26 V UV Hysteresis Input Current UV = 2.5V -10 -20 -29 µA Remote Shutdown Threshold Voltage at UVLO Falling 0.32 0.66 V 110 mV Remote Shutdown Hysteresis Regulation and Over-Voltage Comparators FB Regulation Level Internal Reference Trip Point for Switch ON FB Overvoltage Threshold Trip Point for Switch OFF 1.2 FB Bias Current 1.225 1.25 V 1.62 V 60 nA Switch Characteristics Buck Switch RDS(ON) ITEST = 200mA, BST-SW = 7V Synchronous RDS(ON) ITEST = 200mA Gate Drive UVLO VBST − VSW Rising 1.8 Ω 0.45 1 Ω 3 3.6 V 0.8 2.4 Gate Drive UVLO Hysteresis 260 mV 144 ns Minimum Off-Time Minimum Off-Timer FB = 0V On Time Generator TON Test 1 VIN = 32V, RON = 100k 270 350 460 ns TON Test 2 VIN = 48V, RON = 100k 188 250 336 ns TON Test 3 VIN = 75V, RON = 250k 250 370 500 ns TON Test 4 VIN = 10V, RON = 250k 1880 3200 4425 ns 3 www.ti.com LM34927 FB to RTN ESD Rating (Human Body Model(Note 5) Lead Temperature (Note 2) Storage Temperature Range Absolute Maximum Ratings (Note 1) LM34927 Symbol Parameter Conditions Min Typ Max 0.7 1.02 1.3 Units Current Limit Current Limit Threshold Current Limit Response Time Time to Switch Off OFF-Time Generator (Test 1) OFF-Time Generator (Test 2) A 150 ns FB = 0.1V, VIN = 48V 12 µs FB = 1.0V, VIN = 48V 2.5 µs Thermal Shutdown Temp. 165 °C Thermal Shutdown Hysteresis 20 °C PSOP-8 40 °C/W LLP-8 40 °C/W Thermal Shutdown Tsd Thermal Resistance θJA Junction to Ambient Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics. The RTN pin is the GND reference electrically connected to the substrate. Note 2: For detailed information on soldering plastic PSOP package, refer to the Packaging Data Book available from National Semiconductor Corporation. Max solder time not to exceed 4 seconds. Note 3: All limits are guaranteed by design. All electrical characteristics having room temperature limits are tested during production at TA = 25°C. All hot and cold limits are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control. Note 4: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading. Note 5: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. www.ti.com 4 LM34927 Typical Performance Characteristics Efficiency at 750kHz, VOUT1 = 10V VCC vs VIN 30177905 30177904 VCC vs ICC ICC vs External VCC 30177906 30177907 TON vs VIN and RON TOFF (ILIM) vs VFB and VIN 30177909 30177908 5 www.ti.com LM34927 IIN vs VIN (Operating, Non Switching) IIN vs VIN (Shutdown) 30177910 www.ti.com 30177911 6 LM34927 Block Diagram 30177913 FIGURE 2. Functional Block Diagram 7 www.ti.com LM34927 Functional Description The LM34927 step-down switching regulator features all the functions needed to implement a low cost, efficient, isolated bias supply. This high voltage regulator contains 100V, Nchannel buck and synchronous switches, is easy to implement, and is provided in thermally enhanced PSOP-8 and LLP-8 packages. The regulator operation is based on a constant on-time control scheme using an on-time inversely proportional to VIN. This control scheme does not require loop compensation. Current limit is implemented with forced offtime inversely proportional to VOUT. This scheme ensures short circuit protection while providing minimum foldback. The simplified block diagram of the LM34927 is shown in Figure 2. The LM34927 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator is well suited for 48V telecom and 42V automotive power bus ranges. Protection features include: thermal shutdown, Undervoltage Lockout, minimum forced off-time, and an intelligent current limit. The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated as follows: This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor (COUT). A minimum of 25mV of ripple voltage at the feedback pin (FB) is required for the LM34927. In cases where the capacitor ESR is too small, additional series resistance may be required (RC in Figure 3 Low Ripple Output Configuration). For applications where lower output voltage ripple is required the output can be taken directly from a low ESR output capacitor, as shown in Figure 3 Low Ripple Output Configuration. However, RC slightly degrades the load regulation. Control Overview The LM34927 regulator employs a control principle based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an internal reference (1.225V). If the FB voltage is below the reference the internal buck switch is switched on for the one-shot timer period, which is a function of the input voltage and the programming resistor (RT). Following the on-time the switch remains off until the FB voltage falls below the reference, and the forced minimum offtime has expired. When the FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB voltage is approximately equal to 1.225V (typ). In a synchronous buck converter, the low side (sync) FET is ‘on’ when the high side (buck) FET is ‘off’. The inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’. There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of the output loading. The operating frequency remains relatively constant with load and line variations. The operating frequency can be calculated as follows: VCC Regulator The LM34927 contains an internal high voltage linear regulator with a nominal output of 7.6V. The input pin (VIN) can be connected directly to the line voltages up to 100V. The VCC regulator is internally current limited to 30mA. The regulator sources current into the external capacitor at VCC. This regulator supplies current to internal circuit blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the VCC pin reaches the Undervoltage Lockout threshold of 4.5V, the IC is enabled. The VCC regulator contains an internal diode connection to the BST pin to replenish the charge in the gate drive boot capacitor when SW pin is low. At high input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall achievable output power. As an example, with the input at 48V and switching at high frequency, the VCC regulator may supply up to 7mA of current resulting in 48V x 7mA = 336mW of power dissipation. If the VCC voltage is driven externally by an alternate voltage source, between 8V and 14V, the internal regulator is disabled. This reduces the power dissipation in the IC. 30177917 FIGURE 3. Low Ripple Output Configuration www.ti.com 8 The feedback voltage at FB is compared to an internal 1.225V reference. In normal operation, when the output voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225V. The high side switch will stay on for the on-time, causing the FB voltage to rise above 1.225V. After the on-time period, the high side switch will stay off until the FB voltage again falls below 1.225V. During start-up, the FB voltage will be below 1.225V at the end of each on-time causing the high side switch to turn on immediately after the minimum forced off-time of 144ns. The high side switch can be turned off before the on-time is over, if peak current in the inductor reaches the current limit threshold. On-Time Generator The on-time for the LM34927 is determined by the RON resistor, and is inversely proportional to the input voltage (VIN), resulting in a nearly constant frequency as VIN is varied over its range. The on-time equation for the LM34927 is: See Figure 4 below. RON should be selected for a minimum on-time (at maximum VIN) greater than 100ns, for proper operation. This requirement limits the maximum frequency for each application. Overvoltage Comparator The feedback voltage at FB is compared to an internal 1.62V reference. If the voltage at FB rises above 1.62V the on-time pulse is immediately terminated. This condition can occur if 30177908 FIGURE 4. TON vs VIN and RON The current limit protection feature is peak limited, the maximum average output will be less than the peak. Current Limit The LM34927 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 1.02A the present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is controlled by the FB voltage and the input voltage VIN. As an example, when FB = 0V and VIN = 48V, a maximum off-time is set to 16μs. This condition occurs when the output is shorted, and during the initial part of start-up. This amount of time ensures safe short circuit operation even up to the maximum input voltage of 100V. In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and start-up time. The offtime is calculated from the following equation: N-Channel Buck Switch and Driver The LM34927 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.01uF ceramic capacitor connected between the BST pin and SW pin provides the voltage to the driver during the on-time. During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges from VCC through the internal diode. The minimum off-timer, set to 144ns, ensures a minimum time each cycle to recharge the bootstrap capacitor. Synchronous Rectifier The LM34927 provides an internal synchronous N-Channel MOSFET rectifier. This MOSFET provides a path for the in- 9 www.ti.com LM34927 the input voltage and/or the output load changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225V. Regulation Comparator LM34927 ductor current to flow when the high-side MOSFET is turned off. The synchronous rectifier has no diode emulation mode, and is designed to keep the regulator in continuous conduction mode even during light loads which would otherwise result in discontinuous operation. This feature specifically allows the user to design a secondary regulator using a transformer winding off the main inductor to generate the alternate regulated output voltage. the set-point divider. When the UVLO threshold is exceeded, the current source is activated to quickly raise the voltage at the UVLO pin. The hysteresis is equal to the value of this current times the resistance RUV2. <0.66V Shutdown VCC regulator disabled. Switcher disabled. Under Voltage Detector 0.66V – 1.225V Standby VCC regulator enabled. Switcher disabled. VCC < 4.5V Standby VCC regulator enabled. Switcher disabled. UVLO The LM34927 contains a dual level Undervoltage Lockout (UVLO) circuit. When the UVLO pin voltage is below 0.66V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.66V but less than 1.225V, the controller is in standby mode. In standby mode the VCC bias regulator is active while the regulator output is disabled. When the VCC pin exceeds the VCC undervoltage thresholds and the UVLO pin voltage is greater than 1.225V, normal operation begins. An external set-point voltage divider from VIN to GND can be used to set the minimum operating voltage of the regulator. UVLO hysteresis is accomplished with an internal 20μA current source that is switched on or off into the impedance of VCC >1.225V Mode Description VCC > 4.5V Operating VCC enabled. Switcher enabled. If the UVLO pin is wired directly to the VIN pin, the regulator will begin operation once the VCC undervoltage is satisfied. 30177921 FIGURE 5. UVLO Resistor Setting When activated, typically at 165°C, the controller is forced into a low power reset state, disabling the buck switch and the VCC regulator. This feature prevents catastrophic failures from accidental device overheating. When the junction temperature reduces below 145°C (typical hysteresis = 20°C), the VCC regulator is enabled, and normal operation is resumed. Thermal Protection The LM34927 should be operated so the junction temperature does not exceed 150°C during normal operation. An internal Thermal Shutdown circuit is provided to protect the LM34927 in the event of a higher than normal junction temperature. www.ti.com 10 LM34927 Application Information TYPICAL ISOLATED BIAS APPLICATION SCHEMATIC A typical isolated bias supply application is shown in Figure 6 below. Inductor (L) in a typical buck circuit is replaced with a coupled inductor (X1). A diode (D1) is used to rectify the voltage on the secondary output. The nominal voltage at the secondary output (VOUT2) is given by: where VF is the forward voltage drop of D1, and NP, NS are the number of turns on the primary and secondary of coupled inductor X1. For output voltage (VOUT1) above the maximum VCC (8.3V), the VCC pin can be diode connected to VOUT1 for higher efficiency and low dissipation in the IC. 30177922 FIGURE 6. Typical Isolated Application Schematic 3W ISOLATED BIAS APPLICATION SCHEMATIC A complete 3W bias supply for isolated bias supply application is shown in Figure 7 below. 30177933 FIGURE 7. A 3W Isolated Application Schematic 11 www.ti.com LM34927 in this configuration. If primary loading is required a diode will be required between VOUT primary and VCC. LOWEST PART COUNT ISOLATED APPLICATION SCHEMATIC A low part count schematic for isolated bias application is shown in Figure 8 below. The primary should not be loaded 30177941 FIGURE 8. Lowest Part Count Isolated Application Schematic The capacitive ripple is not in phase with the inductor current. As a result of this, the capacitive ripple does not decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and decreases monotonically during off-time. The resistive ripple must exceed the capacitive ripple at the output node (VOUT) for stable operation. If this condition is not satisfied, unstable switching behavior is observed in COT converters, with multiple on-time bursts in close succession followed by a long off-time. Type 3 ripple method uses Rr and Cr and the switch node (SW) voltage to generate a triangular ramp. This triangular ramp is ac coupled using Cac to the feedback node (FB). Since this circuit does not use the output voltage ripple, it is ideally suited for applications where low output voltage ripple is required. See application note AN-1481 for more details for each ripple generation method. RIPPLE CONFIGURATION LM34927 uses Constant-On-Time (COT) control scheme, in which the on-time is terminated by an on-timer, and the offtime is terminated by the feedback voltage (VFB) falling below the reference voltage (VREF). Therefore, for stable operation, the feedback voltage must decrease monotonically, in phase with the inductor current during the off-time. Furthermore this change in feedback voltage (ΔVFB) during off-time must be large enough to suppress any noise component present at the feedback node. Table 1 shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1 and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output voltage ripple has two components: 1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor. 2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor. www.ti.com 12 Type 2 Reduced Ripple Configuration 13 LM34927 Type 1 Lowest Cost Configuration Type 3 Minimum Ripple Configuration www.ti.com LM34927 Layout Recommendation A proper layout is essential for optimum performance of the circuit. In particular, the following guidelines should be observed: 1. CIN: The loop consisting of input capacitor (CIN), VIN pin, and RTN pin carries switching currents. Therefore the input capacitor should be placed close to the IC, directly across VIN and RTN pins and the connections to these two pins should be direct to minimize the loop area. In general it is not possible to accommodate all of input capacitance near the IC. A good practice is to use a 0.1μF or 0.47μF capacitor directly across the VIN and RTN pins close to the IC, and the remaining bulk capacitor as close as possible (Refer to Figure 9 Placement of Bypass Capacitors). 2. CVCC and CBST: The VCC and bootstrap (BST) bypass capacitors supply switching currents to the high and low 3. 4. side gate drivers. These two capacitors should also be placed as close to the IC as possible, and the connecting trace lengths and loop area should be minimized (See Figure 9 Placement of Bypass Capacitors). The Feedback trace carries the output voltage information and a small ripple component that is necessary for proper operation of LM34927. Therefore care should be taken while routing the feedback trace so avoid coupling any noise to this pin. In particular, feedback trace should not run close to magnetic components, or parallel to any other switching trace. SW trace: SW node switches rapidly between VIN and GND every cycle and is therefore a possible source of noise. SW node area should be minimized. In particular SW node should not be inadvertently connected to a copper plane or pour. 30177940 FIGURE 9. Placement of Bypass Capacitors www.ti.com 14 LM34927 Physical Dimensions inches (millimeters) unless otherwise noted PSOP–8 Outline Drawing NS Package Number MRA08A LLP-8 Outline Drawing (Dimensions in mm) NS Package Number SDC08B 15 www.ti.com www.ti.com LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. 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