APPLICATION NOTE | AN50 |
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I/O Controls
In addition to the Voltage Identification, there are several sig- nals that control the
RC5050 and RC5051 Description
Simple Step-Down Converter
S1 | L1 |
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VIN | D1 | C1 | RL Vout |
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Figure 1. Simple Buck DC-DC Converter
Figure 1 illustrates a step-down DC-DC converter with no feedback control. The derivation of the basic step-down con- verter is the basis for the design equations for the RC5050 and RC5051. Referring to Figure 1, the basic operation begins by closing the switch S1. When S1 is closed, the input voltage VIN is impressed across inductor L1. The current flowing in this inductor is given by the following equation:
IL = | (VIN – VOUT )TON |
where TON is the duty cycle (the time when S1 is closed).
When S1 opens, the diode D1 conducts the inductor
current and the output current is delivered to the load accord- ing to the following equation:
IL = | VOUT(TS – TON ) |
whereTS is the overall switching period and (TS - TON) is the time during which S1 is open.
By solving these two equations, we can arrive at the basic relationship for the output voltage of a
TON
V = V
OUT IN TS
In order to obtain a more accurate approximation for VOUT, we must also include the forward voltage VD across diode D1 and the switching loss, VSW. After taking into account these factors, the new relationship becomes:
()TON VOUT = VIN + VD – VSW
TS
where VSW = MOSFET switching loss
=IL • RDS,ON
The RC5050 and RC5051 Controllers
The RC5050 is a programmable
Table 4. RC5050 and RC5051 Differences
| RC5051 | RC5050 |
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Operation | Synchronous | |
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Package | ||
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Output Enable/ | Yes | Yes |
Disable |
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Main Control Loop
Refer to the RC5051 Block Diagram illustrated in Figure 2. The control loop of the regulator contains two main sections; the analog control block and the digital control block. The analog section consists of signal conditioning amplifiers feeding into a set of comparators which provide the inputs to the digital control block. The signal conditioning section accepts inputs from the IFB (current feedback) and VFB (voltage feedback) pins and sets up two controlling signal paths. The voltage control path amplifies the VFB signal and presents the output to one of the summing amplifier inputs. The current control path takes the difference between the IFB and VFB pins and presents the resulting signal to another input of the summing amplifier. These two signals are then summed together with the slope compensation input from the oscillator. This output is then presented to a comparator, which provides the main PWM control signal to the digital control block.
The additional comparators in the analog control section set the point at which the current limit comparator disables the output drive signals to the external power MOSFETs.
The digital control block takes the comparator inputs and the main clock signal from the oscillator to provide the appropri- ate pulses to the HIDRV and LODRV output pins. These pins control the external power MOSFETs. The digital sec- tion utilizes high speed Schottky transistor logic, allowing the RC5050 and the RC5051 to operate at clock speeds as high as 1MHz.
High Current Output Drivers
The RC5051 contains two identical high current output drivers that utilize high speed bipolar transistors in a
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