Datasheet MAX1771 (Maxim) - 9

MAX1771
12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller
The control circuitry allows the IC to operate in continu- ous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is R2 FB VOUT turned on, it stays on until either 1) the maximum on- time one-shot turns it off (typically 16µs later), or 2) the MAX1771 switch current reaches the peak current limit set by the R1 C5* current-sense resistor. The MAX1771 switching frequency is variable (depend- ing on load current and input voltage), causing variable switching noise. However, the subharmonic noise gen- R1 = 10kΩ TO 500kΩ erated does not exceed the peak current limit times the GND VOUT R2 = R1 ( -1) filter capacitor equivalent series resistance (ESR). For VREF example, when generating a 12V output at 500mA from VREF = 1.5V a 5V input, only 100mV of output ripple occurs using * SEE TEXT FOR VALUE the circuit of Figure 2a. Figure 3. Adjustable Output Circuit
Low-Voltage Start-Up Oscillator
The MAX1771 features a low input voltage start-up oscil- lator that guarantees start-up with no load down to 2V when operating in bootstrapped mode and using inter- R1 and R2 configured as shown in Figure 3. For nal feedback resistors. At these low voltages, the supply adjustable-output operation, select feedback resistor voltage is not large enough for proper error-comparator R1 in the 10kΩ to 500kΩ range. R2 is given by: operation and internal biasing. The start-up oscillator V has a fixed 50% duty cycle and the MAX1771 disre- OUT R2 = (R1) (––––– -1) gards the error-comparator output when the supply volt- VREF age is less than 2.5V. Above 2.5V, the error-comparator where VREF equals 1.5V. and normal one-shot timing circuitry are used. The low- voltage start-up circuitry is disabled if non-bootstrapped For preset-output operation, tie FB to GND (this mode is selected (FB is not tied to ground). forces bootstrapped-mode operation. Figure 2 shows various circuit configurations for boot-
Shutdown Mode
strapped/non-bootstrapped, preset/adjustable operation. When SHDN is high, the MAX1771 enters shutdown mode. In this mode, the internal biasing circuitry is
Determining RSENSE
turned off (including the reference) and VOUT falls to a Use the theoretical output current curves shown in diode drop below VIN (due to the DC path from the Figures 4a–4d to select RSENSE. They were derived input to the output). In shutdown mode, the supply using the minimum (worst-case) current-limit compara- current drops to less than 5µA. SHDN is a TTL/CMOS tor threshold value over the extended temperature logic-level input. Connect SHDN to GND for normal range (-40°C to +85°C). No tolerance was included for operation. RSENSE. The voltage drop across the diode was assumed to be 0.5V, and the drop across the power
Design Procedure
switch rDS(ON) and coil resistance was assumed to be 0.3V.
Setting the Output Voltage
To set the output voltage, first determine the mode of
Determining the Inductor (L)
operation, either bootstrapped or non-bootstrapped. Practical inductor values range from 10µH to 300µH. Bootstrapped mode provides more output current capa- 22µH is a good choice for most applications. In appli- bility, while non-bootstrapped mode reduces the supply cations with large input/output differentials, the IC’s current (see the Typical Operating Characteristics). If a output current capability will be much less when the decaying voltage source (such as a battery) is used, inductance value is too low, because the IC will always see the additional notes in the Low Input Voltage operate in discontinuous mode. If the inductor value Operation section. is too low, the current will ramp up to a high level before The MAX1771’s output voltage can be adjusted from the current-limit comparator can turn off the switch. very high voltages down to 3V, using external resistors The minimum on-time for the switch (tON(min)) is Maxim Integrated 9