MAXREFDES1198

The MAX17632 family of parts (MAX17632A, MAX17632B, and MAX17632C) includes high-efficiency, high-voltage, synchronous step-down DC-DC converters with integrated MOSFETs operating over an input-voltage range of 4.5V to 36V. These parts can deliver current up to 2A. The MAX17632A and MAX17632B are fixed 3.3V and fixed 5V output parts, respectively. The MAX17632C is an adjustable output voltage (0.9V to 90% of V_IN) part. Built-in compensation across the output-voltage range eliminates the need for external compensation components.

The MAX17632 features peak-current-mode control architecture. The device can be operated in forced pulse width modulation (PWM), pulse-frequency modulation (PFM), or discontinuous-conduction mode (DCM) to enable high efficiency under full-load and light-load conditions. Key features of this design include the following:

• Wide 4.5V to 36V Input
• 3.3V Output
• Up to 2A Output Current
• Fully Assembled and Tested
• Proven PCB Layout

This design is a single-output, synchronous buck, stepdown DC-DC converter for small size and low output voltage. Table 1 provides an overview of the design specification.

Hardware Needed for Quick Setup

• 4.5V to 36V, 10A DC input power supply
• MAXREFDES1198 board
• Load capable of sinking 5A
• Digital voltmeter (DVM)

This document provides a detailed systematic technical guide for the design of a buck converter using the MAX17632 for smaller size. Refer to the MAX17632 IC data sheet and MAX175632A EV kit data sheet for device operation details. The converter design has been built and tested, details of which follow later in this document.

The buck power converter is a DC-DC converter whose output voltage is less than the input voltage. This is a non-isolated topology, which means the input and output share a common ground. Figure 2 shows the basic circuit of the synchronous buck power converter. The difference between synchronous buck and traditional buck is that in a synchronous buck, a transistor NL is placed in parallel to the diode to reduce the voltage drop and, therefore, to increase the efficiency.

A buck power converter has the following components:

• Input capacitor (C_IN) and output capacitor (C_OUT)
• A switch, in this case a transistor NH
• An energy storage element, inductor (L)
• Transistor NL and diode D1 to conduct during the off-state of the switch

Figure 3 shows the basic operation of the buck power converter. During the on-state (t_ON) of the transistor NH, the voltage at the node LX is equal to V_IN and then the current across the inductor rises linearly at a rate of (V_IN - V_OUT)/L. When the transistor NH is off (t_OFF), the voltage at the LX node is 0V and the current in the inductor falls linearly. It is the property of the inductor to maintain the flow of the current, so this reverses the polarity of the inductor during the NH off-state. The current through the inductor never falls to zero. This is called Continuous Conduction Mode (CCM). The ripple current ∆I is an important parameter that is approximately 20 percent to 50 percent of I_L(load current).

Now that the theory behind the synchronous buck is explained, a practical design technique can now be illustrated. The design procedure involves the following stages: output voltage selection, inductor and capacitor selection, and setup of the switching frequency. This document is intended to complement the information contained in the MAX17632 IC data sheet.

The following design parameters are used throughout:

Step 1: Setting the Output Voltage

The MAX17632’s output voltage can be adjusted between 0.9V and 0.9 × V_IN. The output using the R3 and R4 resistors connected to the FB pin is calculated as follows:

In this design since the output voltage is 3.3 volts we are considering R3 = 0Ω and R4 = open circuit. Refer to Typical applications circuits in MAX17632 data sheet.

Step 2: Setting the Switching Frequency

The MAX17632 can operate between 400kHz and 2.2MHz. The RT pin is used to set the regulator’s switching frequency. The RT pin is left unconnected, which defaults to 400kHz. This is calculated using the following formula:

where R_RT is in kΩ and f_SW is in kHz. The switching frequency f_SW = 400kHz is chosen here.

Step 3: Selecting the Output Inductor

The LX pin is connected to the switching node of the inductor. The value of the inductor is calculated as follows:

where V_OUT = 3.3V, f_SW = 400kHz, and L = 6.8µH are chosen for this design.

Step 4: Selecting the Output and Soft-Start Capacitor

The soft-start feature ramps up the output voltage slowly, reducing input inrush current during startup. A capacitor connected from SS to SGND determines the soft-start. This soft capacitor depends upon the output capacitor. The output capacitance can be calculated as follows:

where I_STEP is the load current step, t_RESPONSE is the response time of the controller, ∆V_OUT is the allowable output voltage deviation, f_C is the target closed-loop crossover frequency, and f_SW is the switching frequency. Choose f_C to be 1/10 of f_SW because in this design the switching frequency is less than 800kHz (refer to the MAX17632 data sheet for more information).

Substitute the following values in the above equations:

I_STEP = 1A

∆V_OUT = 0.33V

f_C = (400k/10)

f_SW = 400kHz

t_RESPONSE = 8.3µs

C_OUT = 42µF

Hence, two 22µF capacitors are selected in parallel for the nominal value.

The soft-start capacitance (CSS) is calculated as follows:

C_SS ≥ 28 x 10-6 × C_OUT × V_OUT

C_OUT is the selected output capacitance:

C_SS ≥ 28 x 10-6 × 44 × 10-6 × 4 

where C_SS ≥ 4.066nF. C_SS = 5600pF is considered the nominal value.

Step 5: Setting the Undervoltage Lockout (UVLO)

UVLO is a technique used to shut down the power to the IC when the input voltage is less than operational value. R1 and R2 are used to set the UVLO of the converter. In this design the pin is connected to V_IN pins for always-on operation.

Step 6: Compensation

The MAX17632 is internally compensated.

Step 7: Input Capacitor Selector

The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit’s switching. The input capacitor RMS current requirement (I_RMS) is defined by the following equation:

Substituting the values:

I_OUT(MAX) = 2A

V_IN = 24V

V_OUT = 3.3V

I_RMS = 0.688A

Calculate the input capacitance using the following equation:

Substituting the values:

I_OUT(MAX) = 2A

η = 91%

f_SW = 400kHz

∆V_IN = 45mV

where the input capacitor = 2.2µF. As suggested in the data sheet page 19 input capacitor selection an electrolytic capacitor of 47µF is added in parallel.