Datasheet MAX1932 (Maxim) - 7

MAX1932 Digitally Controlled, 0.5% Accurate, Safest APD Bias Supply
Output and DAC Adjustments Range
The inductance value is given by: Many biasing applications require an adjustable output 2 2 voltage, which is easily obtained using the MAX1932’s V D T ( IN) × × × η L = DAC output (Figure 2). I 2 ( ) × V OUT MAX OUT The DAC output voltage is given by the following equation: where VIN is the input voltage, IOUT(MAX) is the maxi- mum output current delivered, VOUT is the output volt- 1 2 . V 5 ⎞ 1 2 . V 5 ⎞ age, and T is the switching period (3.3µs), V CODE η is the DACOUT = × ⎛⎝⎜ 256 ⎠⎟ +⎛⎝⎜ 256 ⎠⎟ estimated power conversion efficiency, and D is the maximum duty cycle: On power-up, DACOUT defaults to FF hex or 1.25V, which corresponds to the minimum VOUT output voltage. D < (V The voltage generated at DACOUT is coupled to FB OUT - VIN)/VOUT up to a maximum of 0.9 through R6. DACOUT can sink only 50µA so: Since the L equation factors in efficiency, for inductor cal- culation purposes, an η of 0.5 to 0.75 is usually suitable. 1 2 . V 5 R6 ≥ For example, with a maximum DC load current of 2.5mA, 50 A μ a 90V output, VIN = 5V, D = 0.9, T = 3.3µs, and η esti- Select the minimum output voltage (VOUTFF), and the mated at 0.75, the above equation yields an L of 111µH, maximum output voltage (VOUT01) for the desired so 100µH would be a suitable value. adjustment range. R5 sets the adjustment span using The peak inductor current is given by: the following equation: V D T IN R5 = (V I = × × OUTFF - VOUT01) (R6/1.25V) PK L These are typical calculations. For worst case, refer to R8 sets the minimum output of the adjustment range the article titled “Choosing the MAX1932 External with the following equation: Indicator, Diode, Current Sense Resistor, and Output Filter Capacitor for Worst Case Conditions” located on the Maxim website in the Application Notes section (visit R8 = (1.25V ✕ R5)/(VOUTFF) www.maxim-ic.com/an1805).
External Power-Transistor Selection Setting the Output Voltage without
An N-FET power switch is required for the MAX1932. The
the DAC
N-FET switch should be selected to have adequate on- Adjust the output voltage by connecting a voltage- resistance with the MOSFET VGS = VIN(MIN). The break- divider from the output (VOUT) to FB (Figure 2 with R6 down voltage of the N-FET must be greater than VOUT. omitted). Select R8 between 10kΩ to 50kΩ. Calculate For higher-current output applications (such as 5mA at R5 with the following equation: 90V), SOT23 high-voltage low-gate-threshold N-FETs may not have adequate current capability. For example, ⎛ VOUT ⎞ with a 5V input, a 90V, 5mA output requires an inductor R5 = R8 −1 ⎝⎜ 1 2 V 5 ⎠⎟ . peak of 240mA. For such cases it may be necessary to simply parallel two N-FETs to achieve the required cur-
Inductor Selection
rent rating. With SOT23 devices this often results in Optimum inductor selection depends on input voltage, smaller and lower cost than using a larger N-FET device. output voltage, maximum output current, switching fre-
Diode Selection
quency, and inductor size. Inductors are typically spec- The output diode should be rated to handle the output ified by their inductance (L), peak current (IPK), and voltage and the peak switch current. Make sure that the resistance (LR). diode’s peak current rating is at least IPK and that its breakdown voltage exceeds VOUT. Fast reverse recov- ery time (trr < 10ns) and low junction capacitance www.maximintegrated.com Maxim Integrated | 7