Design of DC-DC Converter Based on MAX1524

1 Introduction
MAX1524 is a small, step-up DC-DC converter produced by MAXIM, USA. It can be applied to various topologies such as step-up, SEPIC, and flyback. With fixed on-time and minimum off-time, it can provide high efficiency under a wide range of input/output voltage combinations and load currents. The fixed on-time can be selected, which is 0.5μs with a maximum duty cycle of 50% and 3μs with a maximum duty cycle of 85%, making it more convenient to optimize the size and cost of external components over a wide output range. MAX1524 adopts bootstrap operation mode, allowing startup at a lower input voltage. The device has internal soft-start and short-circuit protection functions, and has a fault lock function, which automatically shuts down the controller when a short circuit occurs. The device uses a 6-pin SOT23 package, has a low quiescent current (25μs), and does not require a current detection resistor. It is widely used in digital cameras, LCD monitors, and other low-power electronic products.

2 Working Principle
2.1 Package
The package of MAX1524 is shown in Figure 1.

2.2 Pin Function
—— Pin 1 GND, ground terminal;
—— Pin 2 FB, feedback input, connect an external resistor divider, the feedback value of FB is 1.25V;
—— Pin 3 SET, on-time control, when connected to VCC, it is 3μs, the duty cycle is 85%; when connected to GND, it is 0.5μs, the duty cycle is 50%;
—— Pin 4 SHDN, shutdown control input, high level is normal operation, low level is shutdown state;
—— Pin 5 EXT, external MOSFET drive terminal;
—— Pin 6 VCC, power input terminal, bypassed by a 0.1μF capacitor, the input range is 2.5V~5.5V, using bootstrap working mode, connected in series with the output terminal through a 10Ω resistor.
2.3 Working Principle
MAX1524 generally adopts bootstrap working mode, and its low-voltage startup oscillator ensures that the device starts when the input voltage is lower than 1.5V. The startup oscillator triggers the MOSFET with a 25% duty cycle signal to increase the output voltage. When the output voltage exceeds the undervoltage lockout threshold, the normal control circuit starts to work, and the startup oscillator stops working.
For different output voltage/input voltage combinations, the MAX1524 has an optional fixed on-time to achieve high-efficiency operation. A 3μs on-time is recommended for high boost ratios, which allows a duty cycle of more than 80% and improves operating efficiency. When working at low boost ratios and high frequencies, it is better to use a 0.5μs on-time, which can reduce the size of external inductors and capacitors.

The soft-start function of MAX1524 can reduce the inductor current, input capacitance and inductor force at startup. The soft-start time is 3.2ms and no external components are required. When the soft-start ends, if the output voltage drops below half of the normal regulation value, the device works in the fault lock state, making SHDN low, the device is in the shutdown state, the internal control circuit is turned off, EXT outputs low, the current drops to 1μA, and the output voltage is one diode forward conduction voltage drop lower than the input voltage. When VCC drops below 2.37V, the device is locked by undervoltage.

3 Typical Applications

The MAX1524 can operate in either continuous or discontinuous mode. When the inductor current is not allowed to decay to zero, the device operates in continuous mode, which requires a large inductor to keep the inductor ripple current less than half the input current. The advantages of this mode are low peak current, low resistance loss, and low output ripple. When the inductor is large enough so that the ripple current at maximum load is 30% of the input current, the transient operating characteristics are best and the efficiency is highest. However, under light load, such as when the load is less than 1/6 of the maximum load, it will switch to discontinuous operation.
There are two situations in which the discontinuous mode works: (1) High output voltage, that is, the output voltage/input voltage exceeds the value that can be obtained when the MAX1524 has the maximum duty cycle. When the applied duty cycle exceeds 80% of the maximum duty cycle, the discontinuous mode is required. (2) Low output current. When the maximum output current is very small, using a large inductor is not cost-effective in terms of size and cost. In this case, a small inductor can be used to make it work in discontinuous mode. When the product of the load current and the output voltage/input voltage ratio is less than a few hundred milliamperes, discontinuous mode operation is required.
1) Output voltage setting: Connect the resistor divider between the output and the ground to the FB terminal, R2 is selected between 30kΩ and 100kΩ, and R1 is selected after calculation according to formula (1).

3.1 Design in continuous mode
1) Selection of on-time
For continuous operation, the duty cycle of MAX1524 must be higher than the external maximum duty cycle. When the external duty cycle is higher than 45%, connect SET to GND, and the on-time of 0.5μs makes the opening and closing speed very fast; when applying an 80% duty cycle, connect SET to VCC to obtain a 3μs on-time, and discontinuous mode operation must be used.
2) Determination of switching frequency
In continuous mode, for heavy or medium loads, the switching frequency is independent of the load IL and is related to VIN.
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If the calculated LI is not a standard value, the standard value closest to this calculated value should be selected, and the error can be within ±50%. If the selected inductance value is lower than the calculated ideal value, the peak inductance current will be a little higher, otherwise, the peak inductance current will be a little lower. In view of the high operating frequency, it is recommended to use ferrite cores instead of general cores. The saturation current of the inductor should be higher than the calculated peak current, and its series equivalent resistance (ESR) should be as low as possible. The energy loss of the series equivalent resistance is:

the capacitance value is generally selected to be 2 to 3 times of COUT (MIN). It is recommended not to use ceramic capacitors, but tantalum electrolytic capacitors, etc.
3.2 Design of discontinuous mode
Connect SET to GND so that the maximum duty cycle is less than 67%; connect SET to VCC, the maximum duty cycle is between 67% and 99%, and the discontinuous mode can work at any duty cycle. When the external duty cycle is close to the maximum duty cycle set by SET, the working performance is best.
1) Calculation of switching frequency
The switching frequency is proportional to the load current:

4) Selection of output capacitor
When operating in discontinuous mode, the output capacitor can be selected from low ESR ceramic capacitors or tantalum electrolytic capacitors.

6) Selection of feedback capacitor
For both operating modes, if a feedforward capacitor is used, a larger output ripple will be obtained at the FB end. Therefore, a feedback capacitor needs to be connected between FB and GND. The feedback capacitor value CFB is generally equal to CFF and can be adjusted up and down. If a feedforward capacitor is not used, a feedback capacitor is not required.
7) Selection of input capacitor
The input capacitor CIN can reduce the current peak of the input power supply, improve efficiency, and reduce noise. The impedance value of the input power supply determines the size of CIN. The higher the impedance value, the larger the capacitance. Generally, CIN=COUT is selected.
Considering the size of the capacitor, it is recommended to use a 0.1μF ceramic capacitor. When bootstrapping, a 10Ω resistor is connected between the output terminal and VCC to isolate the input capacitor from the output capacitor.
8) Selection of power MOSFET
N-channel MOSFET needs to be selected, and the main considerations are the total gate charge Qg, reverse transfer capacitance or charge CRSS, on-state resistance RDS(ON), maximum VDS(MAX), and minimum threshold voltage VTH(MIN). When working at high frequency, Qg and CRSS have a greater impact on efficiency and are the main considerations.
9) Selection of diode
When working at high frequency, fast diodes are required. Schottky diodes are recommended because they have fast recovery time and low forward voltage drop. Its current rating should be, and the reverse breakdown voltage must be higher than VOUT. When the output voltage is high, a silicon rectifier diode can be used. [page]

Based on the calculation method of the above parameters, taking the design of a power supply as an example, the parameters are calculated in the continuous working mode, specifically
VIN=3.3V±0.33V, VOUT=5V;
IOUT(MAX)=700mA, IP=1.48A;
Lactual=3.3μH, LI=3.72μH;
CFF=100pF,CFB=100pF,CIN=33μF,
COUT=14μF～448μF,COUT（actual）=33μF;
R1=100kΩ,R2=33.2kΩ
DMAX=45.5％;
tON=0.5μs(SET=GND);
f=761kHz～800kHz(IOUT(MIN)=120mA)
3.3 Typical Application
The typical application circuit of MAX1524 is shown in Figure 2.