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IS32LT3953 Datasheet(PDF) 8 Page - Integrated Silicon Solution, Inc |
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IS32LT3953 Datasheet(HTML) 8 Page - Integrated Silicon Solution, Inc |
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8 / 10 page  IS32LT3953 Integrated Silicon Solution, Inc. – www.issi.com 8 Rev. 00A, 11/29/2013 The minimum output voltage depends on minimum tON and switching frequency. For example, if the minimum tON = 150ns and fSW = 1MHz, then the minimum duty cycle is 15%. That means with VIN = 24V, the minimum VOUT = 3.2V (one LED). To a lesser degree, the output voltage is also affected by other factors such as LED current, on-resistance of the high-side switch, DCR of the inductor, and forward drop of the low-side diode. As a general rule, switching at lower frequencies allows a wider range of VOUT, and hence more flexible LED configurations. This is shown in graph below which shows how the minimum and maximum output voltages vary with LED current (assuming RDS_ON = 0.4Ω, inductor DCR = 0.1Ω, and diode VF = 0.6 V). Figure 3 Minimum and Maximum Output Voltage versus Switching Frequency (V IN = 24V, ILED = 2A, minimum t ON and tOFF = 150ns) If the required output voltage is lower than that permitted by the minimum tON, the controller will automatically extend the tOFF, in order to maintain the correct duty cycle. This means that the switching frequency will drop lower when necessary, while the LED current is kept in regulation at all times. Fault Handling The IS32LT3953 is designed to handle the following faults: Pin-to-ground short Pin-to-neighboring pin short Pin open External component open or short Output short to GND The SW pin overcurrent protection is tripped at around 3.75 A, and the part shuts down immediately. The part then goes through startup retry after approximately 380μs of cool-down period. THERMAL BUDGETING The IS32LT3953 is capable of supplying a 3A current through its high-side switch. However, depending on the duty cycle, the conduction loss in the high-side switch may cause the package to overheat. Therefore care must be taken to ensure the total power loss of package is within budget. For example, if the maximum temperature rise allowed is ∆T = 50K at the device case surface, then the maximum power dissipation of the IC is 1.4 W. Assuming the maximum RDS(on) = 0.4Ω and a duty cycle of 85%, then the maximum LED current is limited to 2A approximately. At a lower duty cycle, the LED current can be higher. |
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