Datasheet LT3507A (Analog Devices) - 10

LT3507A
applications inForMation STEP-DOWN CONSIDERATIONS
1 DCMAX = 1
FB Resistor Network
1+ B The output voltage is programmed with a resistor divider (refer to the Block Diagram) between the output and the where B is the output current capacity divided by the typi- FB pin. Choose the resistors according to: cal boost current from the BOOST pin current vs switch current in the Typical Performance Characteristics section. ⎛ V R1 ⎞ =R2 OUT ⎜ –1⎟ The maximum operating voltage without pulse-skipping ⎝800mV ⎠ is determined by the minimum duty cycle DCMIN: The parallel combination of R1 and R2 should be 10k or VOUT + VF less to avoid bias current errors. VIN(PS) = – VF + VSW DCMIN
Input Voltage Range
with DCMIN = tON(MIN) • fSW. The minimum operating voltage is determined either by Thus both the maximum and minimum input voltages are the LT3507A’s internal undervoltage lockout (4V) or by a function of the switching frequency and output voltages. its maximum duty cycle. The duty cycle is the fraction of Therefore the maximum switching frequency must be set time that the internal switch is on and is determined by to a value that accommodates all the input and output the input and output voltages: voltage parameters and must meet both of the following V criteria for each channel: DC = OUT + VF VIN – VSW + VF ⎛ ⎞ f VOUT +VF MAX1 =⎜⎜ ⎟⎟• 1 where V V t F is the forward voltage drop of the catch diode ⎝ IN(PS) – VSW + VF ⎠ ON(MIN) (~0.4V) and VSW is the voltage drop of the internal switch ⎛ ⎞ (~0.3V at maximum load). This leads to a minimum input fMAX2 =⎜1– VOUT +VF ⎟ voltage of: ⎜ V ⎟• 1 t ⎝ IN(MIN) – VSW + VF ⎠ OFF(MIN) V V OUT + VF The values of t IN(MIN) = – VF + VSW ON(MIN) and tOFF(MIN) are functions of ISW DCMAX and temperature (see chart in the Typical Performance Characteristics section). Worst-case values for switch The duty cycle is the fraction of time that the internal switch currents greater than 0.5A are t is on during a clock cycle. The maximum duty cycle is ON(MIN) = 130ns (for TJ > 125°C t generally given by DC ON(MIN) = 155ns) and tOFF(MIN) = 170ns. MAX = 1– tOFF(MIN)• fSW. However, unlike most fixed frequency regulators, the LT3507A will fMAX1 is the frequency at which the minimum duty cycle not switch off at the end of each clock cycle if there is suf- is exceeded. The regulator will skip ON pulses in order to ficient voltage across the boost capacitor (C3 in Figure 1) reduce the overall duty cycle at frequencies above fMAX1. to fully saturate the output switch. Forced switch off for It will continue to regulate but with increased inductor a minimum time will only occur at the end of a clock current and greatly increased output ripple. The increased cycle when the boost capacitor needs to be recharged. peak inductor current in pulse-skipping will also stress This operation has the same effect as lowering the clock the switch transistor at high voltages and high switching frequency for a fixed off time, resulting in a higher duty frequency. If the LT3507A is allowed to pulse-skip and cycle and lower minimum input voltage. The resultant duty the input voltage is greater than 20V, then the switching cycle depends on the charging times of the boost capaci- frequency must be kept below 1.1MHz to prevent damage tor and can be approximated by the following equation: to the LT3507A. 3507afa 10 For more information www.linear.com/LT3507A Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Package Description Typical Applications Related Parts