MAX1917 Provides Pre-Bias Soft Start for Redundant Supply
MAX1917 was originally designed to meet termination voltage-tracking requirements demanded by double data rate (DDR) memory. To avoid large input inrush current in case a reference step voltage is applied to REFIN, the MAX1917 ramps up the output inductor current in 5 steps up to the preset output current limit threshold. By ramping inductor current directly, and not the feedback reference voltage, the output inductor current is always controlled to only source current to charge the output capacitor until the output voltage reaches its regulation value. Therefore no negative inductor current can be generated during soft start and consequently no discharge of the output capacitor takes place.
Although designed for DDR memory termination supplies, the MAX1917 is a good candidate for a point-of-load power supply controller due to its excellent transient response and small package size. Figure 1 shows a schematic of the MAX1917 for a 2.5V point-of-load application. Table I lists the bill of materials. Figure 2 shows soft start waveforms of both the output voltage and the output inductor current. It is evident from this figure that no discharging of the output voltage occurs during soft start, and the output inductor current is well maintained as a constant current source.
Figure 1. MAX1917 for 2.5V output power supply.
Table 1 Bill of Materials for 2.5V/15A application
|C1||1.0uF/25V ceramic cap, TMK316BJ105ML, 20mW max esr.||Taiyo Yuden|
|C2||4X330uF/25V aluminum caps, ZA series, 26mW max esr.||Rubycon|
|C3||0.47uF/10V ceramic cap, EMK212BJ474MG, 20mW esr max.||Taiyo Yuden|
|C4, C8||0.22uF/10V ceramic capacitor, LMK107BJ224MA||Taiyo Yuden|
|C5||10uF/6.3V ceramic capacitor, JMK316BJ106ML, 20mW max esr.||Taiyo Yuden|
|C6||3X560uF, 4SP560M, 14mW max esr each capacitor.||Sanyo|
|C9||0.22uF/25V ceramic capacitor, TMK312BJ224MF||Taiyo Yuden|
|D1||30V/100mA Schottky diode, CMPSH-3||Central Semi.|
|L1||0.75uH/24A, CDEP149-0R7NC, (esr, 1.2mW max)||Sumida|
Figure 2. Pre-bias soft start waveforms of the MAX1917.