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SP6134H Datasheet(PDF) 6 Page - Exar Corporation |
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SP6134H Datasheet(HTML) 6 Page - Exar Corporation |
6 / 15 page S SP P6 61 13 34 4H H H Hiig gh h V Vo olltta ag ge e,, 6 60 00 0k kH Hz z S Sy yn nc ch hrro on no ou us s P PW WM M S Stte ep p D Do ow wn n C Co on nttrro olllle err © 2008 Exar Corporation 6/15 Rev. 2.0.0 THEORY OF OPERATION GENERAL OVERVIEW The SP6134H is a fixed frequency, voltage mode, synchronous PWM controller optimized for high efficiency. The part has been designed to be especially attractive for split plane applications utilizing 5V to power the controller and 3V to 28V for step down conversion. The heart of the SP6134H is a wide bandwidth transconductance amplifier designed to accommodate Type II and Type III compensation schemes. A precision 0.8V reference present on the positive terminal of the error amplifier permits the programming of the output voltage down to 0.8V via the VFB pin. The output of the error amplifier, COMP, compared to a 1.1V peak-to-peak ramp is responsible for trailing edge PWM control. This voltage ramp and PWM control logic are governed by the internal oscillator that accurately sets the PWM frequency to 300kHz. The SP6134H contains two unique control features that are very powerful in distributed applications. First, Non-synchronous driver control is enabled during start up to prohibit the low side NFET from pulling down the output until the high side NFET has attempted to turn on. Second, a 100% duty cycle timeout ensures that the low side NFET is periodically enhanced during extended periods at 100% duty cycle. This guarantees the synchronized refreshing of the BST capacitor during very large duty ratios. The SP6134H also contains a number of valuable protection features. A programmable input (VIN) UVLO allows a user to set the exact value at which the conversion voltage is at a safe point to begin down conversion, and an internal VCC UVLO ensures that the controller itself has enough voltage to properly operate. Other protection features include thermal shutdown and short-circuit detection. In the event that either a thermal, short-circuit, or UVLO fault is detected, the SP6134H is forced into an idle state where the output drivers are held off for a finite period before a re-start is attempted. SOFT START “Soft Start” is achieved when a power converter ramps up the output voltage while controlling the magnitude of the input supply source current. In a modern step down converter, ramping up the positive terminal of the error amplifier controls soft start. As a result, excess source current can be defined as the current required to charge the output capacitor. IVIN = COUT * DVOUT / DTSoft-start The SP6134H provides the user with the option to program the soft start rate by tying a capacitor from the SS pin to GND. The selection of this capacitor is based on the 10uA pull up current present at the SS pin and the 0.8V reference voltage. Therefore, the excess source can be redefined as: IVIN = COUT * DVOUT *10μA / (CSS * 0.8V) UNDER VOLTAGE LOCK OUT (UVLO) The SP6134H contains two separate UVLO comparators to monitor the bias (VCC) and conversion (VIN) voltages independently. The VCC UVLO threshold is internally set to 4.25V, whereas the VIN UVLO threshold is programmable through the UVIN pin. When the UVIN pin is greater than 2.5V, the SP6134H is permitted to start up pending the removal of all other faults. Both the VCC and VIN UVLO comparators have been designed with hysteresis to prevent noise from resetting a fault. THERMAL AND SHORT-CIRCUIT PROTECTION Because the SP6134H is designed to drive large NFETs running at high current, there is a chance that either the controller or power converter will become too hot. Therefore, an internal thermal shutdown (145°C) has been included to prevent the IC from malfunctioning at extreme temperatures. A short-circuit detection comparator has also been included in the SP6134H to protect against the accidental short or sever build up of current at the output of the power converter. This comparator constantly |
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