Design of high-speed synchronous buck switching regulator based on LM2727 controller

1. Introduction

LM2727 is an N-channel power MOSFET synchronous buck high-speed switching regulator control IC designed by National Semiconductor (NS) for cable modems, set-top boxes/home gateways, DDR core power supplies and high-efficiency distributed power supplies. The LM2727 has an input voltage range of 2.2~16V, an output voltage that can be adjusted down to 0.6V, a load current from 0.7A to 20A, an efficiency of up to 95%, and a switching frequency that can be adjusted from 50kHz to 2MHz using a single resistor. It also provides current limiting, output undervoltage and overvoltage protection, and a Power Good (PG) flag.

2. Structure and characteristics of LM2727

The LM2727 chip integrates oscillators, PWM comparators, feedback error amplifiers, synchronous drive logic, MOSFET high/low-side gate drivers and current limiting, soft start, input undervoltage lockout (UVLO), output undervoltage and overvoltage protection (UVP/OVP) and power good flag circuits. Its internal structure block diagram is shown in Figure 1.

The LM2727 is packaged in a 14-pin plastic TSSOP package, and the pinout is shown in Figure 2.

The functions of each pin of LM2727 are shown in Table 1.

Table 1 LM2727 pin functions

The main features of the LM2727 are: (1) The output voltage can be set by a resistor divider and can be adjusted to a minimum of 0.6V; (2) Current limiting does not require a sensing resistor; (3) The switching frequency, soft-start time, current limit level, etc. are programmable; (4) It provides output UVP/OVP lockout, power good flag and IC logic shutdown. When the output voltage drops below 70% of the normal value or exceeds 118%, the IC is locked out.

3. Principle and design of DC-DC buck regulator based on LM2727

The DC-DC switching buck regulator circuit of the controller LM2727 is shown in Figure 3. The technical requirements of the power supply circuit are:

Input voltage Vin=5V;

Output voltage Vo=1.2V;

Output current Io=10A;

Switching frequency fsw=300kHz;

Efficiency η=85%.

Based on these performance requirements, the design procedures and methods are described as follows.

3.1 Input capacitor selection

The input capacitor Cin1,2 is used to suppress the input current ripple. The input effective current ripple Irms-rip is determined by the output current Io and the switch duty cycle D, and the calculation formula is:

In the above formula: Io=10A; D=Vo/Vin=1.2V/5V=0.24, so Irms-rip=4.27A.

Cin1 and Cin2 can use two Sanyo 10MV5600AX (5600μF/10V) aluminum electrolytic capacitors in parallel. The operating temperature of this capacitor is 105℃, and the equivalent series resistance ESR is 18mΩ at 100kHz. Cin1 and Cin2 can also use MLCC, tantalum capacitors, OSCON, SP and POSCAPS capacitors.

Cin1,2 is used as high-frequency bypass to filter out switching harmonics and input noise, and can be connected using two 1μF/10V ceramic capacitors.

The capacitor Cin and resistor Rin on the Vcc pin are used to smooth the Vcc voltage and the bootstrap voltage. Cin can use a 2.2μF/25V ceramic capacitor, Rin uses a 10Ω resistor, and the bootstrap capacitor Cboot is 0.1μF.

3.2 Input inductor Lin selection

The input inductor Lin is used to suppress switching noise and limit the conversion rate of the input current. Its inductance value is determined by the voltage change ΔV on Lin from no load to full load and the maximum change rate of the input current (di/dt) max, and can be expressed as

Lin≥ΔV/(di/dt)max

ΔV can be considered as the full load current through the input capacitor ESR. Since the total input capacitor RESR = 18mΩ/2 = 9mΩ, ΔV = IoRESR = 10A × 9mΩ = 90mV. In devices such as desktop computers, di/dt is about 0.1A/μs, so Lin ≥ 90mV/(0.1A/μs) = 0.9μH. The input inductor should be able to handle the DC input current, which is

IIN-DC=IOD/η=（10A×0.24）/0.85=2.83A

The input inductor can be selected as TDK SLF12575T-1R2N8R2, which has an inductance of 1.2μH, can handle a DC current of 8.2A, has a DC resistance of 7mΩ, and measures 7.29×7.29×3.51(mm).

3.3 Output Inductor L1 Selection

The output inductor L1 is used to smooth the square wave generated by the switching action and control the output ripple current ΔI. Its inductance value is determined by the input voltage Vin, output voltage Vo, switching frequency fsw, duty cycle D and ΔIo. The calculation formula is:

If the peak-to-peak output current ripple is 40% of the load current, the required inductance value is 1.5μH. The output inductor must be able to handle the peak-to-peak current (i.e., Io+ΔIo/2=10A+2A=12A). Therefore, L1 can use Coilcraft's DO5022-152HC inductor, which has an inductance of 1.5μH, a rated current of 15A, a DC resistance of 4mΩ, and a size of 22.35×16.26×18 (mm).

3.4 Output Capacitor Selection

Output capacitors C01~3 are used to control the output voltage ripple ΔVo and apply load current during rapid load changes. The output ripple voltage ΔVo and ripple current ΔIo determine the maximum allowable ESR, that is,

RESR (max) = ΔVo/ΔIo

In order to maintain a 2% peak-to-peak output voltage ripple and a 40% peak-to-peak inductor current ripple, the maximum ESR value is RESR (max) = 1.2 × 0.2V/10A × 0.4 = 6mΩ. CO1~3 can use Sanyo's 10MV5600AX. Three such capacitors are connected in parallel, with a total capacitance of 16.8mF and a total RESR = 18mΩ/3 = 6mΩ.

3.5 Frequency Setting Resistor Rfadj Selection

The switching frequency of the LM2727 is set by the external resistor Rfadj to ground on the IC pin FREQ. Since fsw = 300kHz is selected, the value of Rfadj is

3.7 Control Loop Compensation Component Selection

RC1, CC1 and CC2 connected between the LM2727 pins EAO and FB form the compensation network of the control loop, which is used to improve the error amplifier DC gain and bandwidth BW. Selecting RC1 = 229kΩ, CO1 = 4.7PF, CO2 = 270PF can obtain a phase angle of 63° and a bandwidth of 29.3kHz.

3.8 Output Voltage Setting Component Selection

The resistor divider Rfb1/Rfb2 connected to the IC feedback pin FB is used to sense the output voltage and also to set the output voltage value. The internal threshold of the IC pin FB is 0.6V, so we can get:

You can choose Rfb1=Rfb2=10kΩ.

3.9 Current Limiting Resistor RCS Selection

LM2727 limits the output current by sensing the voltage on Q2 through the ISEN pin. When the low-side switch Q2 is turned on, a 50μA current flows through RCS inside the IC. The RCS value is:

RCS = RDSON (Q2) × ILIM / 50 μA

In the above formula: RDSON(Q2) is the low-side switch on-resistance; ILIM is the limiting current. If Q2's RDSON(Q2) is 10mΩ, ILIM is 15A (when there is enough margin), RCS is 3KΩ, and a standard resistor of 3.3KΩ can be selected.

3.10 Selection of soft-start capacitor CSS

The external ground capacitor CSS on the LM2727 pin SS is used to set the soft start time tss. The calculation formula is:

CSS=tss/2.5×105

The delay required by microprocessors is generally 3ms, so Css can use a 12nF/25V capacitor, such as Vishay's V71206X123Kxx, with a size of 1206.

3.11 Power MOSFET and Bootstrap Diode Selection

When calculating the current limiting resistor, the current limiting value is set at 15A and the on-resistance of the MOSFET is no more than 10mΩ. Based on this, Si442DY power MOSFETs from Vishay can be selected for Q1 and Q2.

The bootstrap diode D1 connected between Vin and IC pin BOOT can be a 30V BAT-54 Schottky diode.

3.12 Calculation of efficiency

The efficiency of a DC-DC switching regulator depends on the total power loss of the system, and MOSFET power loss dominates the total power loss.

(1) MOSFET power loss

The power loss of MOSFET includes conduction loss, gate charging loss and switching loss.

The conduction loss PCnd of MOSFET is

PCnd=D(Io2RDSONK)+(1-D)(Io2RDSONK)

In the above formula: factor K = 1.3; RDSON of Si4442DY = 4.1mΩ; IO = 10A; D = 0.24. Based on this, we can get: RCnd = 0.533W.

The switching loss Psw of MOSFET is

Psw=0.5VinIo(tr+tf)fsw

In the above formula: Si4442DY's rise time tr=11ns, fall time tf=47ns, Vin=5V, fsw=300kHz, so Psw=0.435W.

The gate charging loss RGC of MOSFET is

RGC=nVccQGSfsw

In the above formula: n is the number of MOSFETs, n=2; QGC is the gate charge, QGC of Si4442DY=36nC. The result calculated by the above formula is PGC=0.108W.

The total power loss of MOSFET PC(Q1,Q2) is

PC(Q1,Q2)=PCnd+PSW+PGC=0.533W+0.435W+0.108W=1.076W

(2) Input capacitor loss

The input capacitor power loss RCin can be calculated according to the following formula:

In the above formula, n is the number of capacitors, n=2; RESR=18mΩ; I2rms-rip=4.27A. The calculated result is PCin=0.082W.

(3) Input inductor loss

The DC input current IIN-DC is

IIN-DC=IOD/η=10A×0.24/0.85=2.82A

The input inductor loss PLin is

PLin=I2IN-DCRDC(Lin)=(2.82A)2×7mΩ=0.055W

(4) Output inductor loss

The output inductor DC resistance RDC (L1) = 4mΩ, and its power loss RL1 is

PL1=I2ORDC(L1)=(10A)2×4mΩ=0.4W

(5) LM2727 chip loss

LM2727 chip voltage VCC = 5V, operating current IQ-VCC = 2mA, power loss PIC is

PIC=IQ-vcc VCC=2mA×5V=0.01W

Furthermore, the output capacitor forms the other half of the buck switching converter power stage and its losses can be ignored when calculating efficiency.

The total system power loss Ptotal is

Ptotal=PC(Q1,Q2)+PCin+PLin+PIC=1.076W+0.082W+0.4W+0.01W=1.568W

The output power of the voltage regulator is PO=IOVO=10A1.2V=12W

The system efficiency η is

η=Po/(Po+Ptotal)=12W/(12W+1.568W)=88.4%

4. Buck switching regulator circuit example

A 500kHz buck regulator circuit with 3.3V input and 0.8V/5A output is shown in Figure 4. The input DC bus voltage Vin of the circuit is 3.3V, the supply voltage VCC of LM2727 is 5V, the system efficiency is 87%, Q1 and Q2 use Si4442DY, and D1 uses BAT-54.

Figure 5 shows a buck regulator circuit that supplies power to the MOSFET gate driver inside the LM2727. This circuit eliminates the bootstrap circuit composed of D1 and Cboot, and the +12V power supply voltage is directly added to the BOOT terminal of 1C. The input voltage Vin of this regulator is 5V, the output is 1.8V/3A, and the switching frequency is 600kHz. Q1/Q2 uses Si4826DY dual N-channel MOSFET.

Figure 6 shows a buck regulator circuit for asymmetric digital subscriber line (ADSL). Since the SS pins of the two LM2727s are connected together, the two ICs start at the same time, the soft start time is 5ms, and the two outputs of 1.8V and 3.3V occur synchronously. If a UV or OV fault occurs in one circuit, both circuits are locked off at the same time. The switching frequency of the circuit is 1.4MHz, and the output is 1A. Q1/Q2 uses Si4826DY in SO-8 package, and D1 uses BAT-54.

5. Conclusion

The LM2727 is a voltage mode high speed synchronous buck regulator PWM controller. This controller with programmable parameters and features power good flag, output shutdown (SD), current limit and UVP/OVP is suitable for applications such as set-top boxes, thin clients, DSL/cable modems, etc.