Datasheet MAX1797 (Maxim) - 11

ОписаниеLow-Supply Current, Step-Up DC-DC Converters with True Shutdown
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Applications Information. Inductor Selection. Capacitor Selection. PC Board Layout and Grounding

Applications Information Inductor Selection Capacitor Selection PC Board Layout and Grounding

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link to page 5 link to page 12 link to page 12 link to page 12 MAX1795/MAX1796/ Low-Supply Current, Step-Up DC-DC Converters MAX1797 with True Shutdown VTRIP is the input voltage where the low-battery detector  I  output goes high impedance. RIP IOUT(MAX) = ILIM + 2    For single-cell applications, LBI may be connected to the battery. When VBATT <1.0V>, the LBI threshold increases to where: IRIP = Inductor ripple current (A) 0.925V (see the Typical Operating Characteristics section). VOUT = Output voltage (V) Connect a pullup resistor of 100kΩ or greater from LBO to I OUT for a logic output. LBO is an open-drain output and LIM = Device current limit (0.25A, 0.5A, or 1A) can be pulled as high as 6V regardless of the voltage at RPFET = On-resistance of P-channel MOSFET OUT. When LBI is below the threshold, the LBO output (Ω) (typ 0.27Ω) is high impedance. If the low-battery comparator is not LESR = ESR of Inductor (Ω) (typ 0.095Ω) used, ground LBI and LBO. VBATT = Input voltage (V)
Applications Information
L = Inductor value in μH
Inductor Selection
tOFF = LX switch’s off-time (μs) (typ 1μs) An inductor value of 22μH performs well in most appli- D = Duty cycle cations. The MAX1795/MAX1796/MAX1797 will also RNFET = On-resistance of N-channel MOSFET work with inductors in the 10μH to 47μH range. Smaller (Ω) (typ 0.17Ω) inductance values typically offer a smaller physical size IOUT(MAX) = Maximum output current (A) for a given series resistance, allowing the smallest overall circuit dimensions, but have lower output current capabil-
Capacitor Selection
ity. Circuits using larger inductance values exhibit higher Table 1 lists suggested tantalum or polymer capacitor output current capability, but are physically larger for the values for typical applications. The ESR of both input same series resistance and current rating. bypass and output filter capacitors affects efficiency and The inductor’s incremental saturation current rating should output ripple. Output voltage ripple is the product of the be greater than the peak switch-current limit, which is peak inductor current and the output capacitor ESR. High- 0.25A for the MAX1795, 0.5A for the MAX1796, and 1A for frequency output noise can be reduced by connecting a the MAX1797. However, it is generally acceptable to bias 0.1μF ceramic capacitor in parallel with the output filter the inductor into saturation by as much as 20% although capacitor. See Table 2 for a list of suggested component this will slightly reduce efficiency. Table 1 lists some sug- suppliers. gested components for typical applications.
PC Board Layout and Grounding
The inductor’s DC resistance significantly affects efficien- Careful printed circuit layout is important for minimizing cy. Calculate the maximum output current (IOUT(MAX)) ground bounce and noise. Keep the IC’s GND pin and the as follows, using inductor ripple current (IRIP) and duty ground leads of the input and output filter capacitors less cycle (D): than 0.2in (5mm) apart. In addition, keep all connections to the FB and LX pins as short as possible. In particular, VOUT + ILIM × (R PFET + LESR ) − BAT V T I when using external feedback resistors, locate them RIP =  L (RPFET + LESR ) as close to FB as possible. To maximize output power  + t   OFF 2  and efficiency and minimize output ripple voltage, use a ground plane and solder the IC’s GND pin directly to the  IRIP  VOUT + ILIM − × (R PFET + LESR ) −   BAT V T ground plane.  2 D  =  IRIP  VOUT + ILIM − × (R PFET − RNFET +   LESR )  2  and Maxim Integrated │

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