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Zakładka z wyszukiwarką danych komponentów |
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Zakładka z wyszukiwarką danych komponentów |
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SC5014 Arkusz danych(PDF) 13 Page - Semtech Corporation |
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SC5014 Arkusz danych(HTML) 13 Page - Semtech Corporation |
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13 / 34 page  SC5014 3 Applications Information (continued) Boost Converter Operation The SC504 includes a boost controller with programma- ble switching frequency. It applies a current-mode control method with an integrated compensation loop as shown in the diagram below. The clock (see block diagram on page) from the oscillator sets the latch and turns on the external power MOSFET, which serves as the main power switch. The current flowing through this switch is sensed by the current sense resistor in series with the switch. The sensed switch current is summed with the slope-com- pensated ramp and fed into the modulating input of the PWM comparator. When the modulating ramp intersects the error amplifier output (COMP), the latch is reset and the power MOSFET is turned off. The sense resistor also sets the peak current limit of the power MOSFET, I OCP using the following equation: I OCP[A] = 0.4 / RCS [Ω] EA Min.Voltage Detection Boost Oscillator Control Logic COMP OSC + + - PWM COMP Slope Comp DC + - VIN=4.5to27V IO C5 L1 Q1 D1 LS CS NDRV RCS The current-mode control system contains two loops. For the inner current loop, the error amplifier (EA) output (COMP) controls the peak inductor current. In the outer loop, the EA regulates the output voltage for driving the LED strings. Boost Converter Switching Frequency Selection The resistor between FSET and GND sets the boost con- verter switching frequency (200kHz to 2.2MHz) using the following equation: f SW [kHz] = 0 5 / R FSET [kΩ] A higher switching frequency allows the use of low-profile height inductors for space-constrained and cost-sensitive applications. Over-Voltage Protection (OVP) The SC504 features programmable output over-voltage protection to prevent damage to the IC and output capaci- tor in the event of a LED string open-circuit. The boost con- verter output voltage is sensed at the OVP pin through the resistor voltage divider. The OVP trip threshold (refer to detailed application circuit on page 23, Figure 2) can be cal- culated using the following equation: OVP Trip Voltage [V] = .2 X (R + R2) / R2 When the OVP pin voltage exceeds .2V, the boost con- verter turns off and the FLT pin is pulled low. When the OVP pin voltage falls below the OVP threshold (falling), the boost converter restarts and the FLT pin is released. There is 0mV hysteresis between the OVP pin threshold (falling) and the OVP pin threshold (rising). This results in an output voltage hysteresis expressed as: Output OVP Hysteresis [mV] = 0 X (R + R2) / R2 LED Current Sink The SC504 provides 4 current sinks and each can sink up to 20mA current. It incorporates LED string short-circuit protection (trip-level programmable; can be disabled) and LED string open-circuit protection. LED String Peak Current Programming LED string peak current (at 00% dimming) can be set by selecting resistor R ISET, connected between ISET and GND. The relationship between R ISET resistance and single LED string peak current is calculated using the following equation: I LED [mA] = 2 X (036 X .23) / RISET [kΩ] The string current can be programmed up to 20mA. Unused Strings The SC504 may be operated with less than 4 strings. In this mode of operation, all unused IO pins should be con- nected to ground. During start-up, these unused strings are detected and disabled while other active strings work normally, and FLT does not get pulled low. |
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