sc473mltrt Semtech Corporation, sc473mltrt Datasheet - Page 10

• Manufacturer Part Number: sc473mltrt
• Description: Sc473 Single-phase Single Chip Graphics Core Power Supply
• Manufacturer: Semtech Corporation

Introduction
The SC473 is a new generation of hysteretic converters
which combines the best features of Semtech’s hysteretic
converter technology with the benefi t of an error amplifi er.
The SC473 provides a complete solution to high perfor-
mance graphics core requirements.
In the SC473, the ripple for the hysteretic switching control
is provided by DCR sensing. This provides several advan-
tages over plain voltage-mode hysteretic converters, which
switch on voltage ripple.
• No current sense resistors are required resulting
• Full differential feedback of the output voltage
• Stable with all-ceramic output
Because the basic control is hysteretic, the SC473 provides
the fastest possible transient response without switching
at very high frequencies. This results in higher effi ciency
with less expensive parts because switching losses are
reduced.
The SC473 also provides a full range of features:
• Graphics functions are implemented on-chip:
• A 2.00V voltage reference is provided
• Separate hysteresis and current limit settings
• A full suite of protection features is provided:
All protection features are latching, and are reset either by
recycling power or toggling the EN signal.
© 2006 Semtech Corp.
POWER MANAGEMENT
Applications Information
in higher converter effi ciency
from the CPU die is enabled
• EN
• PWRGD
• VID programmable Output
• Slew Rate Control
• Power Monitor
• Over-current protection (OCP)
• Fixed and DAC-referenced over-voltage
• Over-temperature protection (OTP)
• Under-voltage detection via PWRGD
protection (OVP)
10
Theory of Operation
Voltage Regulation:
Referring to the block diagram on the preceding page, the
hysteretic comparator is the heart of the converter. The
"FB+" input corresponds roughly to the CMPREF node of
our older generations of IC; the "FB-" input is similar to
CMP.
To regulate, the comparator needs this information:
• DAC (reference) voltage
• Feedback voltage
• Hysteresis voltage
CMPREF receives the reference, voltage feedback and
current information via current sense amplifi er. The refer-
ence source is the DAC. The feedback voltage is received
by the differential amplifi er. A third amplifi er, labeled the
"Error Amplifi er", multiplies the difference between the
"ideal" voltage (DAC) and the actual voltage (FB+ minus
FB-) for faster response. This signal is the reference for
the hysteretic comparator.
CMP has the ripple signal derived from the current sense
inputs plus the hysteresis signal. The DC is stripped from
the ripple signals by the combination of low-pass fi lter and
summing amplifi er.
Current Limit Regulation:
In Current Limit, the voltage hysteretic converter is overrid-
den by the current limit hysteretic comparator, and the TG
pulse is terminated when the output of the current sense
amplifi er reaches the CL_hi threshold and BG is terminated
at the CL_lo threshold. These thresholds are set from the
CLSET resistor divider:
Current limit pulses continue until 32 pulses after the volt-
age droops to the PWRGD low threshold; then the controller
latches off. This current limit algorithm is used in several
generations of V
CL_hi = 0.33 * V(clset)
CL_lo = 0.20 * V(clset)
CORE
controllers and is extremely robust.
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SC473