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Boost, Single-Output, DC-DC Converter: Working & Design Specification

Introduction

The MAX1606 is a step-up DC-DC converter that contains a 0.5A internal power switch along with a 0.5A output isolation switch within an 8-pin uMAX package. The IC operates from a 2.4V to 5.5V supply voltage but can boost battery voltages as low as 0.8V as much as 28V. The MAX1606 utilizes a unique control scheme that provides high efficiency over a number of load conditions. An internal 0.5A MOSFET reduces external component count, along with a high switching frequency (as much as 500kHz) enables tiny surface-mount components. The present limit can be set to 500mA, 250mA, or 125mA, allowing the user to reduce the output ripple and component size in low-current applications. Capabilities include a low quiescent supply current along with a true shutdown mode that saves power by disconnecting the output in the input. The MAX1606 is fantastic for small LCD panels with low current requirements, but could also be employed in other applications.

Main features range from the following:

  • Adjustable Output Voltage Up to 28V
  • 20mA at 20V from one Li+ Battery
  • True Shutdown (Output Disconnected from Input)
  • Output Short-Circuit Protection
  • Up to 88% Efficiency
  • Up to 500kHz Switching Frequency
  • Selectable Inductor Current Limit (125mA, 250mA, or 500mA)
  • 1uA Shutdown Current
  • 8-Pin uMAX Package

Hardware Specification

A single-output boost converter while using MAX1606 is demonstrated for a 1V to 3.3V input voltage range. The output is configured to be equal to 13.5V. The power supply delivers as much as 6mA at 13.5V with as many as 4% output voltage regulation at 1V. Table 1 shows an introduction to the look specification.

Designed–Built–Tested

This document describes the hardware shown in Figure 1. It provides a detailed systematic technical help guide to design in a boost converter using Maxim’s MAX1606 current-mode, step-up DC-DC converter. The maximum efficiency of MAX1606 can be 88%, however, this document describes a design that can turn on in a low input voltage of 1V and deliver a minimum of 6mA load up-to-date with reasonable regulation. The emphasis is on power delivery rather than on efficiency. The advantage of the MAX1606 lies in its simple design and lowest component count, with internal compensation and efficiency at nominal input voltages. The ability supply has been built and tested, details of which follow later within this document.

Table 1. Design Specification

PARAMETER SYMBOL MIN MAX
Input Voltage VIN 1V 3.3 V
Frequency fSW 500 kHz ( variable )
Output Voltage VOUT 13 .5 V
Output Current IOUT 0A 6 mA
Maximum instantaneous

Output Power

POUT 81 mW at 1V

 

The board could be operated from one power supply. To evaluate the board with a single supply, connect a jumper wire in the VIN pad to the VCC pad. Connect a +2.4V (min) to +5.5V (max) power supply towards the VIN or VCC pad. There's two jumpers (JU1 and JU2) on the board that can be configured as shown in Table 2 and Table 3.

Design Technique of Boost Converter Design Using MAX1606

The MAX1606 features a minimum off-time, current-limited control scheme as shown in Figure 2. The job cycle is governed by a pair of one-shots that set the absolute minimum off-time and a maximum on-time. The switching frequency can be as much as 500kHz and is determined by the load and input voltage. The peak current limit of the internal n-channel MOSFET is pin selectable and can be set at 125 mA, 250mA, or 500mA.

Table 2. Jumper JU1 Options

SHUNT LOCATION LIM PIN INDUCTOR CURRENT LIMIT (mA)
1 and 2 Connected to VCC 500
2 and 3 Connected to GND 125
None Floating 250

 

Table 3. Jumper JU2 Options

SHUNT LOCATION SHDN PIN OUTPUT
1 and 2 Connected to VCC ON, VOUT = +13.5V
2 and 3 Connected to GND OFF, VOUT = 0V

 

Step 1: Setting Output Voltage

The output voltage of the converter can be adjusted by connecting a voltage-divider in the output to FB pin. Within this design the output voltage (VOUT) is set to +13.5V by two feedback resistors. The output voltage is determined by the next equation:

The input bias current of FB has a maximum worth of 100nA, that allows large-value resistors for use. For less than 1% error, the current through R2 should be more than 100 times the feedback input bias current

(IFB).

For this design, we selected R2 = 75kΩ. The corresponding value of R1 can be calculated as follows:

The above expression evaluates R1 = 735kΩ. A standard 732 kΩ value is selected as R1: R1 = 732kΩ.

Step 2: Diode Selection

Figure 2. MAX1606 typical inductor current pulses at no load showing minimum off time control scheme

The high switching frequency of 500kHz requires a highspeed rectifier. Schottky diodes with low forward voltage drop are perfect for this design. A 20V single Schottky rectifier (STMicroelectronics STPS0520Z) having a forward voltage drop of 320mV is selected with this design.

Step 3: Current Limit Selection

The MAX1606 allows a selectable inductor current limit of 125mA, 250mA, or 500mA. This allows flexibility in designing for higher current applications or for smaller, compact designs. The low current limit allows the use of a physically smaller inductor in space-constrained low-power applications.

The peak current limit required for the applying can be calculated in the following equation:

where tOFF(MIN) = 0.8us and VBATT(MIN) is the minimum voltage used to supply the inductor. The set current limit should be greater than this calculated value. Also, we have to leave some allowance for losses because of diode drop and also the parasitics within the board.

For example:

VBATT(MIN) = 1V VOUT = 13.5V

IOUT(MAX) = 10mA

For different values of inductors, the variation of peak inductor current requirement is visible from Figure 3.

Step 4: Inductor Selection

Smaller inductance values typically offer smaller physical size for any given series resistance or saturation current. However, circuits using larger inductance values can begin up at lower input voltages and exhibit less ripple, but also provide reduced output power. This happens when the inductance is sufficiently large to avoid the maximum current limit from being reached before the maximum on-time expires. The inductor’s saturation current rating should be greater than the height switching current.

As discussed before, the parasitics in the board and diode drop play a huge role within the quantity of power that could be delivered. Within our design, the cheapest battery voltage that we need to deliver power to the output is 1V. Using large value inductors can begin in the voltage at very low input voltages, however, the quantity of power that could be delivered is highly compromised. Therefore, the inductor selection needs a bit of prototyping the circuit to analyze the output behavior using the selected inductor.

For our design, the specifications are as follows:

V BATT(MIN) = 1V

V OUT = 13.5V

I OUT(MAX) = 6mA t OFF(MIN) = 0.8us

Initially we tested with three inductor values: 10uH, 15uH and 27uH, respectively.

10 uH Testing

To calculate the utmost peak inductor current with this inductor, our design specification in the previous IL(MAX) equation is really as follows:

13.5×6m (13.5 -1)×0.8u

IL(MAX) > +

1 2×10u

IL(MAX) > 581mA

Selecting a 10uH inductor necessitates the peak inductor current to be more than 581m A. Since the device’s maximum inductor current rating is 500mA, the 10uH inductor isn't suitable for this design and that we have to boost the inductance so that peak current requirement is reduced.

15 uH Testing

Similarly, to calculate the utmost peak inductor current with this inductor, input our design specification again in to the previous IL(MAX) expression as follows:

13.5×6m (13.5 -1)×0.8u

IL(MAX) > +

1 2×15u

IL(MAX) > 414mA

A 15uH inductor requires the peak inductor current to become more than 414mA, which is within the device’s current handling capability despite adding some parasitic effects that may increase this requirement on a practical PCB board. The 15uH inductor seems ideal for this design. The prototyped 15uH inductor’s peak inductor current measured is 520mA, as shown in Figure 4, which is showing approximately 20% deviation from the calculated value. The unit is successfully delivering 6mA at 1 V battery voltage in an creation of 13.5V.

27 uH Testing

We used a 27uH inductor to see the result of higher inductance on the output power capability. To calculate the utmost peak inductor current with this inductor, we input our design specification again in to the previous IL(MAX) equation as follows:

13.5×6m (13.5 -1)×0.8u

IL(MAX) > +

1 2×27u

I L(MAX) > 266mA

A 27uH inductor seems suitable for this design as its calculated peak inductor current is well underneath the maximum allowable value of 500mA. The prototyped 27uH inductor’s peak inductor current measured is 348mA as shown in Figure 5, which is again showing approximately 20% deviation from the calculated value. However, power delivering capability is extremely compromised because high inductance doesn’t allow the current to achieve its maximum allowable value prior to the on time elapses. The output voltage degrades to 12.7V at 6mA. The device isn't successfully delivering 6mA at 1V battery voltage at an output of 13.5V.

On the foundation of above analysis, we can conclude that 15uH inductor is easily the most optimum selection, which not just permits the required power delivery to the load, but additionally allows the circuit to turn on in the required minimum VIN of 1V. The inductor selected with this design is Würth Elektronik’s’ 7447779115. It has an inductance value of 15uH with an ISAT of two.2A.

Step 5: Output Capacitor Selection

Selecting the right output capacitor, COUT, and its related ESR is essential for minimizing output voltage ripple. For many applications, use a small 1uF ceramic surface-mount output capacitor. For small ceramic capacitors, the output ripple voltage is covered with the capacitance value.

Step 6: Output Power vs. VIN (MIN) Trade-Off

The output power supplied is principally limited by the reduced input voltage of the battery. As discussed before, the job cycle is controlled by a pair of one-shots that set the absolute minimum off-time along with a maximum on-time. The switching frequency could be as much as 500kHz and depends upon the burden and input voltage. Thus, power delivery towards the load at a certain inductor value could be increased by increasing the worth of V IN(MIN) available. This ought to be evaluated through the measurements. Measurements demonstrated that to provide 10mA with this current design, we should boost the available V IN(MIN) value to 1.2V.

Design Resources

Courtesy: www.maximintegrated.com