Datasheet LT8609, LT8609A, LT8609B (Analog Devices) - 16

LT8609/LT8609A/LT8609B
APPLICATIONS INFORMATION
LT8609A/LT8609B will reduce switching frequency as nec- than the maximum expected output load of the applica- essary to maintain control of inductor current to assure tion. In addition, the saturation current (typically labeled safe operation. ISAT) rating of the inductor must be higher than the load The LT8609/LT8609A/LT8609B is capable of maximum current plus 1/2 of in inductor ripple current: duty cycle approaching 100%, and the VIN to VOUT 1 dropout is limited by the R I Δ DS(ON) of the top switch. In L(PEAK) =ILOAD(MAX) + L 2 this mode the LT8609/LT8609A /LT8609B skips switch cycles, resulting in a lower switching frequency than pro- where ∆IL is the inductor ripple current as calculated sev- grammed by R eral paragraphs below and I T. LOAD(MAX) is the maximum output load for a given application. For applications that cannot allow deviation from the pro- grammed switching frequency at low V As a quick example, an application requiring 1A output IN/VOUT ratios use the following formula to set switching frequency: should use an inductor with an RMS rating of greater than 1A and an ISAT of greater than 1.3A. To keep the V efficiency high, the series resistance (DCR) should be less V OUT + VSW(BOT) – V IN(MIN) = 1– f SW(BOT) + VSW(TOP) than 0.04Ω, and the core material should be intended for SW • tOFF(MIN) high frequency applications. where VIN(MIN) is the minimum input voltage without skipped cycles, V The LT8609/LT8609A/LT8609B limits the peak switch current OUT is the output voltage, VSW(TOP) and V in order to protect the switches and the system from overload SW(BOT) are the internal switch drops (~0.4V, ~0.25V, respectively at max load), f faults. The top switch current limit (I SW is the switching frequency LIM) is typically 4.75A at (set by RT), and t low duty cycles and decreases linearly to 4.0A at D = 0.8. The OFF(MIN) is the minimum switch off- time. Note that higher switching frequency will increase inductor value must then be sufficient to supply the desired the minimum input voltage below which cycles will be maximum output current (IOUT(MAX)), which is a function of dropped to achieve higher duty cycle. the switch current limit (ILIM) and the ripple current:
Inductor Selection and Maximum Output Current
IOUT(MAX) =ILIM – ΔIL 2 The LT8609/LT8609A/LT8609B is designed to minimize solu- The peak-to-peak ripple current in the inductor can be tion size by allowing the inductor to be chosen based on the calculated as follows: output load requirements of the application. During overload or short circuit conditions the LT8609/LT8609A/LT8609B V ⎛ ⎞ OUT VOUT safely tolerates operation with a saturated inductor through ΔIL = ⎜1– ⎟ L • f ⎜ V ⎟ the use of a high speed peak-current mode architecture. SW ⎝ IN(MAX) ⎠ A good first choice for the inductor value is: where fSW is the switching frequency of the LT8609/ LT8609A/LT8609B, and L is the value of the inductor. V L OUT + VSW(BOT) = Therefore, the maximum output current that the LT8609/ fSW LT8609A/LT8609B will deliver depends on minimum the where f switch current limit, the inductor value, and the input SW is the switching frequency in MHz, VOUT is the output voltage, V and output voltages. The inductor value may have to be SW(BOT) is the bottom switch drop (~0.25V) and L is the inductor value in μH. increased if the inductor ripple current does not allow sufficient maximum output current (IOUT(MAX)) given the To avoid overheating and poor efficiency, an inductor switching frequency, and maximum input voltage used in must be chosen with an RMS current rating that is greater the desired application. Rev. I 16 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts