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SC4524AEVB Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4524AEVB Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 19 page  SC4524A 9 Applications Information Operation The SC4524A is a constant-frequency, peak current- mode, step-down switching regulator with an integrated 28V, 2.6A power NPN transistor. Programmable switching frequency makes the regulator design more flexible. With the peak current-mode control, the double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop. This simplifies loop compensation and achieves fast transient response with a simple Type-2 compensation network. As shown in Figure 2, the switch collector current is sensed with an integrated 6.mW sense resistor. The sensed current is summed with a slope-compensating ramp before it is compared with the transconductance error amplifier (EA) output. The PWM comparator trip point determines the switch turn-on pulse width. The current-limit comparator ILIM turns off the power switch when the sensed signal exceeds the 20mV current-limit threshold. Driving the base of the power transistor above the input power supply rail minimizes the power transistor saturation voltage and maximizes efficiency. An external bootstrap circuit (formed by the capacitor C and the diode D in Figure ) generates such a voltage at the BST pin for driving the power transistor. Shutdown and Soft-Start The SS/EN pin is a multiple-function pin. An external capacitor (4.7nF to 22nF) connected from the SS pin to ground sets the soft-start and overload shutoff times of the regulator (Figure 3). The effect of V SS/EN on the SC4524A is summarized in Table . Table 1: SS/EN operation modes Pulling the SS/EN pin below 0.2V shuts off the regulator and reduces the input supply current to 8µA (V IN = 5V). Condition Fault Protective Action IL>ILimit, VFB>0.8V Over current Cycle-by-cycle limit at programmed frequency IL>ILimit, VFB<0.8V Over current Cycle-by-cycle limit with frequency foldback VSS/EN Falling SS/EN<1.9V Persistent over current or short circuit Shutdown, then retry (Hiccup) Table 1 Tj>160C Over temperature Shutdown SS/EN Mode Supply Current <0.2V Shutdown 18uA @ 5Vin 0.4V to 1.23V Not switching 2mA 1.23V to 2.1V Switching & hiccup disabled >2.1V Switching & hiccup armed Load dependent Condition Fault Protective Action IL>ILimit, VFB>0.8V Over current Cycle-by-cycle limit at programmed frequency IL>ILimit, VFB<0.8V Over current Cycle-by-cycle limit with frequency foldback VSS/EN Falling SS/EN<1.9V Persistent over current or short circuit Shutdown, then retry (Hiccup) Table 1 Tj>160C Over temperature Shutdown SS/EN Mode Supply Current <0.2V Shutdown 18uA @ 5Vin 0.4V to 1.23V Not switching 2mA 1.23V to 2.1V Switching & hiccup disabled >2.1V Switching & hiccup armed Load dependent When the SS/EN pin is released, the soft-start capacitor is charged with an internal .6µA current source (not shown in Figure 3). As the SS/EN voltage exceeds 0.4V, the internal bias circuit of the SC4524A turns on and the SC4524A draws 2mA from V IN. The .6µA charging current turns off and the 2µA current source I C in Figure 3 slowly charges the soft-start capacitor. The error amplifier EA in Figure 2 has two non-inverting inputs. The non-inverting input with the lower voltage predominates. One of the non-inverting inputs is biased to a precision V reference and the other non-inverting input is tied to the output of the amplifier A . Amplifier A producesanoutputV = 2(VSS/EN -.23V).V is zero and COMP is forced low when V SS/EN is below .23V. During start up, the effective non-inverting input of EA stays at zero until the soft-start capacitor is charged above .23V. Once V SS/EN exceeds .23V, COMP is released. The regulator starts to switch when V COMP rises above 0.4V. If the soft-start interval is made sufficiently long, then the FB voltage (hence the output voltage) will track V during start up. VSS/EN must be at least .83V for the output to achieve regulation. Proper soft-start prevents output overshoot. Current drawn from the input supply is also well controlled. Overload / Short-Circuit Protection Table 2 lists various fault conditions and their corresponding protection schemes in the SC4524A. Table 2: Fault conditions and protections As summarized in Table , overload shutdown is disabled during soft-start (V SS/EN<2.V). In Figure 3, the reset input of the overload latch B 2 will remain high if the SS/EN voltage is below 2.V. Once the soft-start capacitor is charged above 2.V, the output of the Schmitt trigger B goes high, the reset input of B 2 goes low and hiccup becomes armed. Condition Fault Protective Action IL>ILimit, VFB>0.8V Over current Cycle-by-cycle limit at programmed frequency IL>ILimit, VFB<0.8V Over current Cycle-by-cycle limit with frequency foldback VSS/EN Falling SS/EN<1.9V Persistent over current or short circuit Shutdown, then retry (Hiccup) Table 1 Tj>160C Over temperature Shutdown SS/EN Mode Supply Current <0.2V Shutdown 18uA @ 5Vin 0.4V to 1.23V Not switching 2mA 1.23V to 2.1V Switching & hiccup disabled >2.1V Switching & hiccup armed Load dependent Condition Fault Protective Action IL>ILimit, VFB>0.8V Over current Cycle-by-cycle limit at programmed frequency IL>ILimit, VFB<0.8V Over current Cycle-by-cycle limit with frequency foldback VSS/EN Falling SS/EN<1.9V Persistent over current or short circuit Shutdown, then retry (Hiccup) Table 1 Tj>160C Over temperature Shutdown SS/EN Mode Supply Current <0.2V Shutdown 18uA @ 5Vin 0.4V to 1.23V Not switching 2mA 1.23V to 2.1V Switching & hiccup disabled >2.1V Switching & hiccup armed Load dependent |
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