1.5MHz dual buck in MAX8529 and xDSL modems
1 Overview MAX8529 is a new 1.5MHz step-down controller launched by Maxim in the United States. The two independent regulators in this chip operate at 180 ° C out of phase, thereby reducing the input filtering requirements and reducing the EMI The efficiency is improved, at the same time, the component loss is effectively reduced, and the space of the circuit board is saved. In addition, the chip also has the advantages of low loss and small size, and the output noise in the DS band is very low. The MAX 8529 is mainly used in xDSL modems, broadband routers, DSPs, ASICs and FPGA power supplies. MAX8529 uses the low-side resistance of MOSFET as a current sensing device, thus eliminating the current sensing resistor, which makes the DC-DC structure protected from damage when the output is overloaded or short-circuited. During the short circuit, the power loss can be reduced by adjusting the feedback current threshold. Because MAX8529 contains a power-on reset output, it can send a reset signal to the system when the two outputs reach the adjustment threshold. The main features of MAX8529 are as follows: ◠When working in the DS frequency band, the output noise is very low; ◠Ceramic capacitors can be used as input / output capacitors; ◠Able to perform step-down operation or step-up / step-down operation at the same time; ◠It has four outputs and 90 ℃ phase output (using two MAX 8529); ◠With feedback current limit function; ◠The input voltage is 4.75V ~ 23V; ◠The output voltage is 0V ~ 18V (up to 6A); ◠The efficiency is greater than 90%; ◠Use fixed frequency for PWM operation; ◠With 600kHz ~ 1.5MHz adjustable switching frequency; ◠With external SYNC input; ◠Clock output with main / auxiliary circuit synchronization; ◠With soft start and soft stop functions; ◠The minimum delay of RST output is 140ms; ◠With no loss current limit (no induction resistance) function; ◠Adopt 24-QOP package; ◠Operating temperature is -40 ～ + 85 ℃ (MAX8529EEG). 2.1 The internal structure of MAX8529 The internal structure block diagram of MAX8529 is shown in Figure 1. It consists of PWM comparator, slope regulator, oscillator, 5V linear regulator, soft-start DAC and undervoltage lockout. The controller can get two outputs from the input of 4.75V to 23V, and each output can be adjusted within the range of 1V to 18V. Using synchronous 180 ° C out-of-phase output operation can reduce the input voltage ripple and the total RMS input current ripple. An external resistor can be used to adjust the switching frequency (600k ~ 1.5MHz). To achieve one of two options, the controller can synchronize with an external clock generated by another MAX 8529 or a system clock. A MAX8529 can produce an in-phase or 90 ℃ phase difference output. Since the synchronous clock signal is generally provided by another controller, the two controllers can operate in two phases or four phases (each phase is shifted by 90 ℃). )operating. In addition, the device also has soft start and soft stop functions. The MAX8529 buck converter can provide a PWM voltage mode control for each phase output (see Figure 1). Because it can be divided by the internal oscillator, it can also be driven by an external clock to drive the SYNC input to generate a clock signal. Therefore, The switching frequency of each controller is equal to half of the oscillation frequency (fSW = fOS / 2). In addition, the internal transconductance error amplifier can generate a complete error voltage at the COMP terminal, and can also provide a high-precision DC output. A PWM comparator and a ramp generator can be used to determine the duty cycle at the COMP terminal. On the rising edge of each clock, the high-side MOSFET of REG1 is turned on and remains in this state until the appropriate duty cycle or maximum duty cycle is reached. The second high-side MOSFET (REG2) is turned on at the falling edge of the clock. During each high-side MOSFET turn-on, the corresponding induced current ramps up. In the second half of the switching cycle, the high-side MOSFET turns off and the low-side MOSFET turns on, the induced current ramps down, the energy is released, and the output current is provided. Under overload conditions, when the induced current exceeds the selected valley current threshold, at the corresponding clock edge, the high-side MOSFET still does not conduct, while the low-side MOSFET remains conductive so that the induced current continues to fall. 2.2 Pin Description of MAX8529 The pin arrangement of MAX8529 is shown in Figure 2. The functions of each pin are as follows: 1, 12 feet (COMP2, COMP1): Compensation terminal of regulator 2, 1 respectively; 2, 11 feet (FB2, FB1): It is the feedback input terminal of the regulator 2, 1 respectively, these two pins are connected to a resistor divider between REG2, REG1 and GND, which can be used to adjust the output voltage ( 1V ～ 18V). 3, 10 feet (ILIM2, ILIM1): It is the current-limiting regulation end of regulators 2 and 1, respectively. If you connect ILIM2, ILIM1 to VL, respectively, then the default value of the current limiting threshold of PGND-LX2 and PGND-LX1 is 100mA. By connecting a resistor RILIM2 and RILIM1 between the terminals of ILM2 and ILM1 to GND respectively, the current-limiting thresholds VITH2 and VITH1 of REG2 and REG1 can be adjusted to 50mV (RILM2 = 100kΩ) to 300mV (RILM2) 4 feet (OSC): Oscillation input terminal. Since the controller generates a clock signal by dividing the frequency of the oscillator, the switching frequency is equal to half of the synchronization frequency (fSW = fOSC / 2). Connecting a resistor ROSC between OSC and GND can produce a switching frequency of 600kHz? ROSC = 10kΩ? ~ 1500kHz? ROSC = 4kΩ ?. When connecting an external clock to SYNC, the controller still requires ROS, and selecting ROS should satisfy half of the input of SYNC. 5 feet (V +): input power supply voltage (4.75V ~ 23V). 6 feet (REF): 2V reference output. When used, it should be connected to the GND through a 0.22μF or larger ceramic capacitor. 7 feet (GND): simulated ground. Pin 8 (CKO): clock output, used for external 2-phase or 4-phase synchronous clock output. 9 feet (SYNC): Synchronous or optional clock input. SYNC has three modes of operation, connecting SYNC to a clock of 1,200kHz to 2,800kHz can be externally synchronized; connecting SYNC to GND can be used as a two-phase operation of the main controller; connecting SYNC to VL can be used as a four-phase operation of the main controller . Pins 14 and 23 (DH1, DH2) are the output terminals of the high-side gate driver of the regulator 1/2 respectively. DH1 generally changes between LX1 and BST1, while DH2 usually changes between LX2 and BST2. 15, 22 feet (LX1, LX2): the connection terminals of the external sensor of the regulator 1/2, respectively, you can connect LX1 / LX2 to the switch terminal of the sensor, so that LX1 and LX2 can be used as lower input sources Drive the high-side gate driver of DH1 and DH2. 16, 21 feet (BST1, BST2): It is the booster fast capacitor connection terminal of regulator 1/2 respectively. When designing, an external ceramic capacitor should be connected between BST1 to LX1 and BST2 to LX2. 17, 20 feet (DL1, DL2): respectively the output terminals of the regulator 1/2 low-side gate driver, the output voltage of DL1, DL2 generally varies between PGND and VL. 18 feet (PGND): power ground. Pin 19 (VL): Internal 5V linear regulator output. 24 feet (EN): high-level enable terminal. When this terminal is logic low, the two controllers in the circuit will be turned off. When the terminal is connected to VL, the operation will be maintained. The application circuit of MAX8529 is shown in Figure 3. The energy for all functions of the MAX 8529 comes from the low-fading 5V regulator. The maximum input voltage (V +) of the regulator is 23V, and a 4.7μF ceramic capacitor should be connected between the output (VL) and the PGND. Since the nominal value of the decay voltage of VL is 500mV, when the value of V + is greater than 5.5V, VL is 5V. When VL is lower than 4.5V, the undervoltage lockout circuit will lock the two regulators. The VL linear regulator can output more than 50mA to the IC to stimulate the low-side gate driver to charge the external boost capacitor and supply a small external load. The gate drive voltage of the high-side switch is provided by a fast capacitor boost circuit (see the circuit shown in Figure 3). After starting, the capacitor will charge to 5V. In the second half cycle, the low-side MOSFET turns off, and an internal switch of BST- and DH- is turned off, and the high-side MOSFET is turned on at the same time. SYNC has two functions: one is to synchronize the clock output (CKO) selected from the controller; the other is to synchronize the clock input with the external clock signal of the MAX 8529. Therefore, MAX8529 has two functions of master and slave controllers. The function of CKO is to provide a synchronous clock signal as the switching frequency of MAX8529. It can keep the same phase with the synchronization signal of the controller (SYNC = GND)? It can also have a phase of 90 ° C (SYNC = VL). The three control modes that MAX8529 can provide are as follows: First, when SYNC is connected to GND, the output frequency of CKO is equal to the switching frequency of REG1 (fCKO = fDH1), and the two signals are in phase. At this time, if the signal is synchronized with the slave controller, 2-phase operation can be provided. Second, when SYNC is connected to VL, the output frequency of CKO is equal to twice the switching frequency of REG1 (fCKO = 2fDH1). At this time, the phase difference between the two signals is 90 ° C, so that the system signal can be provided when it is synchronized with the slave controller. 4-phase operation. Thirdly, when the SYNC is driven by an external oscillator, it can be seen that the SYNC input signal is divided by 2 to generate the controller's clock (fSW = fSYCN / 2). At this time, the periodic conversion of REG1 will be performed on the rising edge of the internal clock signal. fCKO = fDH1, and in phase. However, at this time, ROSC is still required, and the internal oscillation frequency at this time is half of the synchronous frequency (fOSC = fSNYC / 2). Because the two independent regulators inside the MAX 8529 use synchronized 180-degree out-of-phase operation output, compared with the traditional in-phase output, its input filtering requirements are reduced, EMI is reduced, and efficiency is greatly improved. It also reduces component losses and saves board space. Especially in the DSL frequency band, its output noise is very low, so it is widely used in the fields of xDSL modem, broadband router, DSP, ASIC and FPGA power supply.
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figure 1
2 The internal functions and pin description of MAX8529 
image 3
Pin 13 (RST): Power-on reset terminal. When the two output voltages are more than 10% below the regulation point, RST is low. After the system completes the soft start, the two output voltages exceed 90% of the normal output voltage (VFB-> 0.9V), then the system will change the RST to a high-impedance state after a delay of 140ms, and maintain the two outputs Maintain a high-impedance state when stable. Connect a resistor between RST and logic input to generate logic level.
Figure 4
3 The application circuit of MAX8529
4 Conclusion 