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RC5031
PRODUCT SPECIFICATION
2
P
Functional Description
The RC5031 contains a precision trimmed zero TC voltage
reference, a constant-on-time architecture controller, a high
current switcher output driver, a low offset op-amp, and
switches for selecting various output modes. The block
diagram in Figure 1 shows how the RC5031 in combination
with the external components achieves a switchable power
supply.
Switch-Mode Control Loop
The main control loop for the switch-mode converter con-
sists of a current conditioning amplifier and one of the two
voltage conditioning amplifiers that take the raw voltage and
current information from the regulator output, compare them
against the precision reference and present the error signal to
the input of the constant-on-time oscillator. The two voltage
conditioning amplifiers act as an analog switch to select
between the internal resistor divider network (set for 3.5V)
or an external resistor divider network (adjustable for 1.5V
to 3.6V.) The switch-mode select pin determines which of
the two amplifiers is selected. The current feedback signals
come across the Iout sense resistor to the IFBH and IFBL
inputs of the RC5031. The error signals from both the
current feedback loop and the voltage feedback loop are
summed together and used to control the off-time duration of
the oscillator. The current feedback error signal is also used
as part of the RC5031 short-circuit protection.
High Current Output Drivers
The RC5031 switching high current output driver (SDRV)
contains high speed bipolar power transistors configured in a
push-pull configuration. The output driver is capable of
supplying 0.5A of current in less than 100ns. The driver’s
power and ground are separated from the overall chip power
and ground for added switching noise immunity.
Internal Reference
The reference in the RC5031 is a precision band-gap type
reference. Its temperature coefficient is trimmed to provide a
near zero TC. For guaranteed stable operation under all load-
ing conditions, a 0.1
m
F capacitor is recommended on the
VREF output pin.
Constant-On-Time Oscillator
The RC5031 switch-mode oscillator is designed as a fixed
on-time, variable off-time oscillator. The constant-on-time
oscillator consists of a comparator, an external capacitor, a
fixed current source, a variable current source, and an analog
switch that selects between two threshold voltages for the
comparator. The external timing capacitor is alternately
charged and discharged through the enabling and disabling
of the fixed current source. The variable current source is
controlled from the error inputs that are received from the
current and voltage feedback signals. The oscillator off-time
is controlled by the amount of current that is available from
the variable current source to charge the external capacitor
up to the high threshold level of the comparator. The on-time
is set be the constant current source that discharges the exter-
nal capacitor voltage down to the lower comparator
threshold.
Figure 1. RC5031 Block Diagram
SWITCHER
SELECT
I
O
I
ON
CONSTANT ON-TIME
OSCILLATOR
gm
gm
V/I
V
H
V
L
V/I
ANALOG
SWITCH
VOSW
To CPU Core
+5V
VREF
65-5031-07
gm
VCC2DET
SWCTRL
From CPU
+12V
REF