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SC1404ITSTR Datasheet(PDF) 9 Page - Semtech Corporation |
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SC1404ITSTR Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 25 page  9 2004 Semtech Corp. www.semtech.com SC1404 POWER MANAGEMENT Detailed Description The SC1404 is a versatile multiple-output power supply controller for battery operated systems. The SC1404 provides synchronous rectified buck control in fixed frequency forced-continuous (PWM) mode and hysteretic PSAVE mode, for two switching power supplies over a wide load range. Out of phase switching reduces input noise and RMS current, which reduces the input filter inductors and capacitors. The two switchers have on-chip preset output voltages of 5.0V and 3.3V. The control circuitry for each PWM controller includes digital softstart, voltage error amplifier with built-in slope compensation, pulse width modulator, power save, overcurrent, overvoltage and undervoltage fault protection. Two linear regulators and a precision reference voltage are also provided. The 5V/30mA linear regulator (VL) uses battery power to feed the gate drivers. For improved efficiency, VL automatically switches over the +5V converter output if available. The 12V linear regulator supplies up to 200mA. Semtech’s proprietary Virtual Current SenseTM provides greater advantages in the aspect of stability and signal-to-noise ratio than the conventional current sense method. PWM Control There are two separate PWM control blocks for each switcher. They are switched out-of-phase with each other. This interleaved topology reduces input filter requirements by reducing current drawn from the filter capacitors. To avoid both switchers switching at the same instance, there is a built-in delay between the turn-on of the 3.3V switcher and 5V switcher, the amount of which depends on the input voltage (see Out-of-Phase Switching). The PWM provides two modes of control over the entire load range. The SC1404 operates in forced continuous conduction mode as a fixed frequency peak current mode controller with falling edge modulation. Current sense is done differently than in conventional peak current mode control. Semtech’s proprietary Virtual Current SenseTM emulates the necessary inductor current information for proper functioning of the IC. In order to accommodate a wide range of output filters, a COMP pin is also available for compensating the error amplifier externally. A nominal error amplifier gain of 18 improves the system loop gain and the output transient behavior. When operating in continuous conduction mode, the high-side mosfet is turned on at the start of each switching cycle. It is turned off when the desired duty cycle is reached. Active shoot-through protection will delay the lower mosfet turn-on until the phase node drops below 1V. The low-side mosfet remains on until the beginning of the next switching cycle. Again, active shoot-through protection ensures that the low-side mosfet gate voltage drops low before the high-side mosfet turns on. Under light load conditions when the PSAVE# pin is low, the SC1404 operates hysteretically in the discontinuous conduction mode to reduce its switching frequency and switching bias current. The switching of the output mosfet does not depend on a given oscillator frequency, but on the hysteretic voltage set around the nominal output voltage. When entering PSAVE# mode (from heavy to light load), if the minimum (valley) inductor current measured at CSHx-CSLx is below the Zero Cross threshold (typically 5 mV) for four switching cycles, the virtual current sense circuitry will shut off and the mode changes to hysteretic mode. As the load current increases, if the minimum (valley) inductor current is above the threshold for four switching cycles, the converter stops psave mode and enters PWM operation. The change in frequency between hysteretic psave and PWM mode provides hysteresis to inhibit chattering between the two modes of operation. Gate Drive / Control The gate drivers on the SC1404 are designed to switch large mosfets. The high-side gate driver must drive the gate of high-side mosfet above the V+ input. The supply for the gate drivers is generated by charging a boostrap capacitor from the VL supply when the low-side driver is on. In continuous conduction mode, the low-side driver output that controls the synchronous rectifier in the power stage is on when the high-side driver is off. Under light load conditions when PSAVE# pin is low, the inductor ripple current will approach the point where it reverses polarity. This is detected by the low-side driver control and the synchronous rectifier is turned off before the current reverses, preventing energy drain from the output. The low-side driver operation is also affected by various fault conditions as described in the Fault Protection section. Internal Bias Supply The VL linear regulator is a 5V output that powers the gate drivers, 2.5V reference and internal controls of the SC1404. The regulator is capable of supplying up to 30mA (including mosfet gate charge current). The VL pin should be bypassed to GND with 4.7uF to supply the large gate drive current pulses. The regulator receives input power from the V+ battery input. Effi- ciency is improved by providing a bootstrap for the VL output. When the 5V SMPS output voltage reaches 5V, internal circuitry turns on a PMOS device between CSL5 and VL. The internal VL regulator is then disabled, and VL bias is provided by the high efficiency 5V switcher. The REF output is accurate to +/- 2% over temperature. It is capable of delivering 5mA max and should be bypassed with 1uF minimum capacitor. Loading the REF pin will reduce the REF voltage slightly as seen in the following table. |
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