Datasheet LT3508 (Analog Devices) - 9

LT3508
APPLICATIONS INFORMATION Setting the Output Voltage
where VF is the forward voltage drop of the catch diode (~0.4V) and V The output voltage is programmed with a resistor divider SW is the voltage drop of the internal switch between the output and the FB pin. Choose the 1% resis- (~0.4V at maximum load). tors according to: Example: ISW = 1.5A and IBOOST = 50mA, VOUT = 3.3V, β ⎛ SW = 1.5A/50mA = 30, DCMAX = 1/(1+1/30) = 96%: V ⎞ R1= R2 OUT ⎜ – 1⎟ ⎝0.8V ⎠ V – 0.4V + 0.4V = 3.8V IN(MIN) = 3.3V + 0.4V 96% R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram.
Maximum Operating Voltage
The maximum operating voltage is determined by the
Minimum Operating Voltage
Absolute Maximum Ratings of the VIN and BOOST pins, The minimum operating voltage is determined either by and by the minimum duty cycle: the LT3508’s undervoltage lockout or by its maximum duty DC cycle. If V MIN = tON(MIN) • f IN1 and VIN2 are tied together, the undervoltage lockout is at 3.7V or below. If the two inputs are used where tON(MIN) is equal to 130ns (for TJ > 125°C tON(MIN) separately, then VIN1 has an undervoltage lockout of 3.7V is equal to 150ns) and f is the switching frequency. or below and VIN2 has an undervoltage lockout of 3V or Running at a lower switching frequency allows a lower below. Because the internal supply runs off VIN1 , chan- minimum duty cycle. The maximum input voltage before nel 2 will not operate unless VIN1 > 3.7V. The duty cycle pulse skipping occurs depends on the output voltage and is the fraction of time that the internal switch is on and is the minimum duty cycle: determined by the input and output voltages: + VF + V V = VOUT IN(PS) – VF + VSW DC = VOUT F DCMIN V + V IN – VSW F Example: f = 790kHz, V Unlike many fi xed frequency regulators, the LT3508 can OUT = 3.3V, DCMIN = 130ns • 790kHz = 0.103: extend its duty cycle by turning on for multiple cycles. The LT3508 will not switch off at the end of each clock V = 3.3V + 0.4V – 0.4V + 0.4V = 36V cycle if there is suffi cient voltage across the boost capaci- IN(PS) 0.103 tor (C3 in Figure 1). Eventually, the voltage on the boost capacitor falls and requires refreshing. Circuitry detects The LT3508 will regulate the output current at input voltages this condition and forces the switch to turn off, allowing greater than VIN(PS). For example, an application with an the inductor current to charge up the boost capacitor. This output voltage of 1.8V and switching frequency of 1.5MHz places a limitation on the maximum duty cycle as follows: has a VIN(PS) of 11.3V, as shown in Figure 2. Figure 3 shows operation at 18V. Output ripple and peak inductor current DCMAX = 1 have signifi cantly increased. Exceeding VIN(PS) is safe if 1+ 1 the output is in regulation, if the external components have β SW adequate ratings to handle the peak conditions and if the where βSW is equal to the SW pin current divided by the peak inductor current does not exceed 3.2A. A saturating BOOST pin current as shown in the Typical Performance inductor may further reduce performance. Do not exceed Characteristics section. This leads to a minimum input VIN(PS) during start-up or overload conditions (for outputs voltage of: greater than 5V, use VOUT = 5V to calculate VIN(PS)). For + operation above 20V in pulse skipping mode, program V V F IN(MIN) = VOUT – VF + VSW the switching frequency to 1.1MHz or less. DCMAX 3508fd 9 Document Outline FEATURES DESCRIPTION APPLICATIONS 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 REVISION HISTORY TYPICAL APPLICATION RELATED PARTS