RC5042 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
RC5042 Datasheet PDF : 20 Pages
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RC5042
PRODUCT SPECIFICATION
Application Information
Simple Step-Down Converter
controller will sense the load level and switch between the
two operating modes automatically, thus optimizing its effi-
ciency under all loading conditions.
S1
L1
+
+5V
VIN
D1
C1
RL Vout
–
65-AP42-01
Figure 2. Simple Buck DC-DC Converter
Figure 2 illustrates a step-down DC-DC converter with no
feedback control. The derivation of the basic step-down
converter will serve as a basis for the design equations for
the RC5042. 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
=
(---V----I--N-----–-----V----O----U----T---)--T----O----N--
L1
Where TON is the duty cycle (the time when S1 is closed).
When S1 opens, the diode D1 will conduct the inductor
current and the output current will be delivered to the load
according to the equation:
IL
=
V-----O----U---T----(--T----S----–-----T---O----N----)-
L1
Where TS 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 step-down converter:
VOUT
=
VI
N
æ
è
T---T-O---S-N--øö
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:
VOUT
=
(VIN
+
VD
–
VS
W
)
-T---O----N--
TS
–
VD
Overview
The RC5042 is a programmable DC-DC controller IC.
When designed around the appropriate external components,
the RC5042 can be configured to deliver more than 14.5A of
output current. During heavy loading conditions, the
RC5042 functions as a current-mode PWM step-down regu-
lator. Under light loads, the regulator functions in the PFM
(pulse frequency modulation), or pulse skipping mode. The
A
CEXT OSCILLATOR
PWM/PFM
Control
VCCQP
HIDRV B
GNDP
D
VO
C
E
A
CEXT
B
HIDRV
C
ILOAD
D
E
Figure 3. Typical Switching Waveforms
Main Control Loop
Refer to the Block Diagram on page 1. The control loop of
the regulator contains two main sections, the analog control
block and the digital control block. The analog block con-
sists of signal conditioning amplifiers feeding into a set of
comparators which provide the inputs to the digital 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 ampli-
fier. 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 thresholds of where the RC5042 enters its pulse skipping
mode during light loads as well as the point at which the
maximum current comparator disables the output drive sig-
nals to the external power MOSFETs.
The digital control block is designed to take the comparator
inputs along with the main clock signal from the oscillator
and provide the appropriate pulses to the HIDRV output
pin that controls the external power MOSFET. The digital
section was designed utilizing high speed Schottky transistor
logic, thus allowing the RC5042 to operate at clock speeds
as high as 1MHz.
8
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