MAX8794(2006) 查看數據表（PDF） - Maxim Integrated
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MAX8794 Datasheet PDF : 13 Pages
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Low-Voltage DDR Linear Regulator
The MAX8794 features an open-drain PGOOD output
that transitions high 2ms after the output initially reach-
es regulation. PGOOD goes low within 10µs of when
the output goes out of regulation by ±150mV. The
MAX8794 features current- and thermal-limiting circuitry
to prevent damage during fault conditions.
3.3V Bias Supply (VCC)
The VCC input powers the control circuitry and provides
the gate drive to the pass transistor. This improves effi-
ciency by allowing VIN to be powered from a lower sup-
ply voltage. Power VCC from a well-regulated 3.3V
supply. Current drawn from the VCC supply remains rel-
atively constant with variations in VIN and load current.
Bypass VCC with a 1µF or greater ceramic capacitor as
close to the device as possible.
VCC Undervoltage Lockout (UVLO)
The VCC input UVLO circuitry ensures that the regulator
starts up with adequate voltage for the gate-drive cir-
cuitry to bias the internal pass transistor. The UVLO
threshold is 2.55V (typ). VCC must remain above this
level for proper operation.
Power-Supply Input (IN)
IN provides the source current for the linear regulator’s
output, OUT. IN connects to the drain of the internal
n-channel power MOSFET. IN can be as low as 1.1V,
minimizing power dissipation. The input UVLO prohibits
operation below 0.8V (typ). Bypass IN with a 10µF or
greater capacitor as close to the device as possible.
Reference Input (REFIN)
The MAX8794 regulates OUTS to the voltage set at
REFIN, making the MAX8794 ideal for memory applica-
tions where the termination supply must track the sup-
ply voltage. Typically, REFIN is set by an external
resistive voltage-divider connected to the memory sup-
ply (VDDQ) as shown in Figure 1.
The maximum output voltage of 1.5V is limited by the
gate-drive voltage of the internal n-channel power
transistor.
Buffered Reference Output (REFOUT)
REFOUT is a unity-gain transconductance amplifier that
generates the DDR reference supply. It sources and
sinks greater than 5mA. The reference buffer is typically
connected to ceramic bypass capacitors (0.33µF to
1.0µF). REFOUT is active when VREFIN > 0.45V and
VCC is above VUVLO. REFOUT is independent of SHDN.
Shutdown
Drive SHDN low to disable the error amplifier, gate-
drive circuitry, and pass transistor (Figure 2). In shut-
down, OUT is terminated to GND with an 8Ω MOSFET.
REFOUT is independent of SHDN. Connect SHDN to
VCC for normal operation.
Current Limit
The MAX8794 features source and sink current limits to
protect the internal n-channel MOSFETs. The source-
and-sink MOSFETs have a typical 3A current limit (1.8A
min). This current limit prevents damage to the internal
power transistors, but the device can enter thermal
shutdown if the power dissipation increases the die
temperature above +165°C (see the Thermal-Overload
Protection section).
Soft-Start Current Limit
Soft-start gradually increases the internal source current
limit to reduce input surge currents at startup. Full-
source current limit is available after the 200µs soft-start
timer has expired. The soft-start current limit is given by:
ILIMIT(SS)
=
ILIMIT ×
tSS
t
where ILIMIT and tSS are from the Electrical Charac-
teristics. Figure 3 shows the MAX8794 PGOOD and
soft-start waveform.
Thermal-Overload Protection
Thermal-overload protection prevents the linear regulator
from overheating. When the junction temperature
exceeds +165°C, the linear regulator and reference
buffer are disabled, allowing the device to cool. Normal
operation resumes once the junction temperature cools
by 15°C. Continuous short-circuit conditions result in a
pulsed output until the overload is removed. A continuous
thermal-overload condition results in a pulsed output. For
continuous operation, do not exceed the absolute maxi-
mum junction-temperature rating of +150°C.
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