SC4525E/F Application Note Converting SC4525C/D to SC4525E/F AN-PM-1302 POWER MANAGEMENT This application note services two purposes. First, it is intended to assist designers in the transition from Semtech’s SC4525C and SC4525D step-down switching regulators to the SC4525E and SC4525F devices. Second, it provides a new external synchronization schematic for SC4525 product family. The SC4525E and SC4525F regulators share the identical footprint as the “C” and “D” versions, so existing customers will not need to make any PCB changes. There are no external component changes necessary to ensure the SC4525E and SC4525F perform identically to the SC4525C and SC4525D. V IN D1 10V – 28V C4 4 .7 µF 1N4148 C1 0 .3 3 µF L1 BST IN SW S C 4525C /E S S /E N 5 .2 µH OUT R4 3 3 .2 k 5 V /3 A FB COMP C7 10nF C8 10pF ROSC GND D2 20BQ 030 R5 1 6 .5 k R7 1 7 .4 k R6 8 .2 5 k C2 1 0 µF X 3 C5 1 .5 n F L 1 : C o iltro n ics C D 1 -5 R 2 C 2 : M u ra ta G R M 3 1 C R 6 0 J1 0 6 K C 4 : M u ra ta G R M 3 2 E R 7 1 H 4 7 5 K Figure 1 — SC4525C/E Typical Application Circuit Schematic V IN D1 10 V – 16 V C4 10µF 1N 4148 C1 0.33µF L1 BST IN SW S C 4525D /F S S /E N 10µH OUT R4 33 .2 k 5 V /3 A FB COMP C7 22 nF C8 47pF RSET R7 11 .5k GND R5 60 .4k D2 20 B Q 030 R6 8.25k C2 47µF C5 2 .2 nF L1: C oiltronics C D 1- 100 C 2: M urata G R M 31 C R 60J476 M C 4 : M urata G R M 31 C R 61 E 106 K Figure 2 — SC4525D/F Typical Application Circuit Schematic Rev. 2.0 © 2013 Semtech Corporation SC4525E/F Application Note Bootstraping the power transistor For VOUT > 8V applications using the SC4525C/D a diode, D4 as shown in Figure 3, is required in parallel with the bootstrap capacitor. There is an improvement in the SC4525E/F circuits. There is no need of that extra diode for Vout >8V applications. This is shown in Figure 4. D1 BST V IN D4 D1 BST C1 V O U T >8V SW IN SC4525C/D GND V IN C1 V O U T >8V SW IN SC4525E/F D2 D2 GND Figure 3 — SC4525C/D VOUT > 8V Application Circuit Figure 4 — SC4525E/F VOUT > 8V Application Circuit Electrical Characteristics Changes The specification differences between the SC4525C/D and SC4525E/F are minor and have minimal or no impact on the circuit performance. The comparison results are shown in Tables 4 and 5. Table 4 — SC4525C and SC4525E Electrical Characteristic Comparison SC4525C Parameter Minimum Minimum Switch On-time (ns) Typical SC4525E Maximum Minimum 150 Typical Maximum 150 250 Table 5 — SC4525D and SC4525F Electrical Characteristic Comparison SC4525D Parameter Minimum Minimum Switch On-time (ns) AN-PM-1302 Typical 150 SC4525F Maximum Minimum Typical Maximum 150 250 SC4525E/F Application Note POWER MANAGEMENT Converting SC4525C/D to SC4525E/F AN-PM-1302 External Synchronization D1 V IN There is no external clock buffer inside the SC4525E. However, the SC4525E may be synchronized to an external clock and synchronization actually works well for regulators running below 35% duty cycle. IN To allow for free-running frequency tolerances, the nominal free-running frequency should be set (using Table 1) to the synchronizing frequency: I)5((581 | I6<1& Synchronization Test 1. Set the SC4525E free-running frequency equal to the clock frequency fSYNC. 2. Start with a single coupling capacitor [Figure 5(a)], test for synchronization with a 10pF coupling capacitor. If the clock cannot lock onto the internal oscillator, then increase CC as needed. If the clock duty cycle can be adjusted, then set it in middle of its locking range. 3. Use the circuit in Figure 5(b) if a single coupling capacitor cannot result in synchronization or there are excessive PWM jitters. 4. Verify synchronization with fFREE-RUN programmed to 0.8fSYNC and 1.2fSYNC. Adjust clock duty cycle if necessary. This ensures frequency locking under the worst-case freerunning frequency tolerances. VOUT SW SS/EN SC4525E/F FB To synchronize the SC4525E, the clock is fed into the ROSC pin through a coupling capacitor CC [Figures 5(a) and 5(b)]. The falling edge of the clock injects a current pulse into the ROSC pin. This current pulse momentarily increases the internal oscillator capacitor charging current and trips the oscillator comparator, thus locking the clock onto the internal oscillator. CC is typically between 4.7pF and 47pF. The clock logic level can be up to 5V. The magnitude of the injected current pulse is proportional to the transition rate of the clock falling edge. CC can be 10pF if the clock swings from 0 to 5V with fall time less than 20ns. The ability to synchronize may depend on the clock duty cycle. The small Schottky diode BAT54 in Figure 5(b) reduces coupling from the clock rising edge to the ROSC pin. It not only extends the useful duty cycle range of the synchronizing clock but also reduces PWM (SW falling edge) jitters. C1 BST CC D2 ROSC COMP GND External C lock R3 (a) D1 V IN IN C1 BST VOUT SW SS/EN SC4525E/F FB BAT 54 CC D2 ROSC COMP GND R3 External C lock (b) Figure 5. Methods of Synchronizing the SC4525E. (a) A Single Ceramic Coupling Capacitor (b) A Coupling Capacitor in Series with a Schottky Diode. Table 1: Resistor for Typical Switching Frequency Freq. (k) ROSC (k) Freq. (k) ROSC (k) Freq. (k) ROSC (k) 200 110 700 25.5 1400 9.76 250 84.5 800 21.5 1500 8.87 300 69.8 900 18.2 1600 8.06 350 57.6 1000 15.8 1700 7.15 400 49.9 1100 14.0 1800 6.34 500 38.3 1200 12.4 1900 5.62 600 30.9 1300 11.0 2000 5.23 © 2013 Semtech Corporation SC4525E/F Application Note Converting SC4525C/D to SC4525E/F AN-PM-1302 POWER MANAGEMENT Frequently Asked Questions Q1. Does my oscillator resistor value need to change? A. No. The switching frequency of SC4525E/F is set with an external resistor from the ROSC pin to ground. The oscillator frequency setting resistor values of the SC4525C/D and SC4525E/F are same. When converting from the SC4525C/D to SC4525E/F, no change of the oscillator resistor is needed. Q2. Do I need to change my compensation network component values? A. No. The switching regulators in the SC4525C/D and SC4525E/F require a simple Type-2 compensation network for stable operation. The correct calculation of these component values (R7, C5 and C8) is very important to maintain the stability of the circuit. It is essential to verify loop compensation by checking regulator load transient response. With the largest load step pertinent to the application applied, the regulator output voltage and the load current were observed. These transient waveforms should not show any ringing or excessive overshoot. It is required to adjust the component values until we get a stable operation. While verifying the load transient response of SC4525E/F, no excessive ringing or overshoot was noticed. So no component change is needed while converting SC4525C/D to SC4525E/F. Q3. If I change SC4525C/D with SC4525E/F, will it impact the efficiency? A. Efficiency is more or less same with both parts. Q4. Do I need to change feedback resistor? A. No, same feedback resistor can be used for both the parts. Q5. Do I need to change feedback resistor? A. No, same feedback resistor can be used for both the parts. © 2013 Semtech Corporation SC4525E/F Application Note © Semtech 2013 All rights reserved. Reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. The information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. No liability will be accepted by the publisher for any consequence of its use. Publication thereof does not convey nor imply any license under patent or other industrial or intellectual property rights. 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