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Texas Instruments Limiting DDR Termination Regulators’ Inrush Current Application notes
Application Report
SNVA758 – August 2016
Limiting DDR Termination Regulators’ Inrush Current
Kaisar Ali
ABSTRACT
The output voltage of DDR termination regulators tends to rise quickly after their VDDQ line is enabled.
Most DDR terminators are specifically designed for fast start-up. They also require bulky output capacitors
for a stable output voltage. This often results in a significant inrush current from DDR terminator voltage
supply to charge the output capacitors and to provide current to the resistive loads. Inrush current in DDR
terminators like LP2996, LP2997, LP2998 can preclude proper operation of the load circuitry. This
application report addresses simple methods to achieve a monotonic, inrush current limited start-up in
LP299x devices. It also describes the selection of output capacitor and option to replace the expensive
electrolytic and tantalum capacitors with ceramics and ESR to compensate the regulator control loop.
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Contents
Overview .....................................................................................................................
Inrush Current ...............................................................................................................
System Reset Caused by the Inrush Current ............................................................................
Solution 1: VDDQ Soft-Start ................................................................................................
Solution 2: Soft-Start with RC Filter Implementation ...................................................................
VTT Capacitor Guidelines for LP299x Devices ..........................................................................
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Overview
The simplified block diagram of Figure 1 depicts basic functional blocks of LP299x devices. These devices
support DDRI, DDRII, DDRIII and DDRIIIL VTT bus termination with VDDQ minimum of 1.35V. LP299x
devices consist of high-speed operational error and reference amplifiers to provide excellent response to
load transients.
VDDQ
SD
AV IN
PVIN
50k
VREF
+
-
50k
+
VTT
VSENSE
GND
Figure 1. Simplified Functional Block Diagram
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Inrush Current
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Inrush Current
When VDDQ is enabled, the error amplifier senses that the output voltage is low and drives the pass
element as hard as possible. The pass element pulls a large inrush current from the AVIN/PVIN power
supply to charge the output capacitance and/or load abruptly after the push pull circuitry is activated.
Inrush current is more common in DDR applications with large VTT capacitors. In most DDR memory
termination circuitries, switching regulators are used to provide AVIN/PVIN voltage rails. The amount of
inrush current often exceeds the switching regulator’s maximum current limit which causes the supply
voltage to droop and activates switching regulator’s UVLO (if available).
There is a misconception about taming the inrush current in DDR termination regulators by using
switching regulators for PVIN/AVIN with soft-start implementation. This doesn’t always help in reducing
inrush current because in most DDR memory applications the power supply for AVIN/PVIN is already
enabled (sequenced up) before the VDDQ rail is enabled. Figure 1 shows the implementation of LP2998 in
DDRII termination configuration. LP299x devices are commonly used in automotive cluster designs to
terminate the DDR memory and are capable of sourcing and sinking 1.5-A constant current.
LP2998
VREF = 0.9V
VREF
SD
SD
+
VDDQ = 1.8V
CREF
VDDQ
AV IN = 3.3V or 5.5V
AV IN
PVIN = 3.3V
VSENSE
PVIN
+
CIN
VTT = 0.9V
VTT
GND
+
COUT = 220 uF
Figure 2. LP2998 DDRII Configuration
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System Reset Caused by the Inrush Current
In this example the designer is using a fairly large capacitor at VTT (VDDQ/2 output). The AVIN/PVIN is
3.3 V provided by LM43001, 1-A buck converter. The 1.8-V VDDQ come from another regulator with very
fast slew rate.
Figure 3 shows the startup of LP2998 device when VDDQ voltage line is enabled using a fast slew rate
(~60 µs). The input current from LM46001 reaches ~5 A which exceeds the buck converter’s current limit
and consequently the input voltage droops down. The LM43001 has under voltage lock out (UVLO) and it
shuts off, causing the whole system to restart.
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Limiting DDR Termination Regulators’ Inrush Current and Capacitor
Selection
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System Reset Caused by the Inrush Current
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Figure 3. System Shutdown Due to Inrush Current
With no inrush-control circuitry in place, the input current is clamped to the regulator’s current limit. When
the AVIN/PVIN supply voltage droops, it signals the UVLO trigger. This causes the entire system to
restart. Figure 4 shows an instrument cluster resetting over and over due to similar issue.
Figure 4. Instrument Cluster DDRII Fault Condition Due to Inrush Current
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Solution 1: VDDQ Soft-Start
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Solution 1: VDDQ Soft-Start
VDDQ is the input used to create the internal reference voltage for regulating VTT. The reference voltage is
generated from a resistor divider of two internal 50-kΩ resistors. This ensures that VTT will track VDDQ / 2
precisely
The following relationship predicts the DDR terminator’s required input current at start-up for a given VTT
rise time, TRISE, where COUT is the termination regulator’s output capacitance.
V
I INRUSH = COUT ´ TT
TRISE
(1)
Thus, we can see an inverse relationship between the VTT voltage rise time and inrush current. In LP299x
DDR terminations, the VTT rise time depends on VDDQ slew rate. Therefore, the simplest solution can be
achieved by implementing a soft-start for VDDQ power supply. If VDDQ start-up slew rate is ≥300 µs, the
inrush current can be reduced by 90% as shown in Figure 5.
Figure 5. Inrush Current Controlled by Increasing VDDQ Slew Rate
The output current in Figure 5 is 500 mA, and VDDQ is enabled with 300-µs slew rate. The inrush current
is reduced from ~6 A in Figure 5 to less than 0.3 A, which is a significant improvement.
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Solution 2: Soft-Start with RC Filter Implementation
In some cases the system designers have very little to no control over the VDDQ voltage supply slew rate,
whether using linear or switching regulators. Some step down voltage regulators don’t have soft start
feature. VDDQ voltage source requires only 18uA current to enable the DDRII termination voltage.
Therefore placing an RC filter at VDDQ pin can conveniently increase the output voltage slew rate,
allowing a slow rise in capacitor charge current. In order to keep the VDDQ voltage losses minimum, the
resistor value should be chosen carefully. Figure 6 shows DDRII terminator configuration using LM2998
with RC filter to minimize the amount of inrush current.
4
Limiting DDR Termination Regulators’ Inrush Current and Capacitor
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Solution 2: Soft-Start with RC Filter Implementation
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LP2998
Lm4301 = 3.3V
VDDQ= 1.5
SD
VREF = 0.9V
VREF
SD
R1
100
+
VDDQ = 1.8V
CREF
VDDQ
C1
10uF
AV IN = 3.3V
AV IN
VSENSE
PVIN = 3.3V
PVIN
+
CIN
VTT = 0.9V
VTT
GND
+
COUT = 220uF
Figure 6. Inrush Current Controlled by RC Filter
In Figure 6, R1 is 100-Ω resistor which keeps the VDDQ supply voltage losses down to 1.8 mV, since the
current through VDDQ is only 18 µA for DDRIII configuration. Figure 7 shows the rise time of the VDDQ
voltage with the additional RC circuitry for I OUT = 500 mA. The measured rise time is slightly above 2
ms.
Figure 7. Inrush Current Controlled by RC Filter
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VTT Capacitor Guidelines for LP299x Devices
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VTT Capacitor Guidelines for LP299x Devices
6.1
Electrolytic and Tantalum Capacitors
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A bypass capacitor should be placed on the VTT line for stability. However, the size of VTT capacitor will
not affect stability, but larger values will improve the transient response and should be sized according to
the design requirements. Electrolytic and tantalum capacitors have parasitic resistance known as ESR.
Most linear regulators have stable range of ESR values which the output capacitor must meet to ensure
stable regulator operation. This makes electrolytic and tantalum more effective in linear regular based
devices. However, their cost is significantly higher than ceramic capacitors which makes them a difficult
choice in cost sensitive designs. Good news is that the unique design of LP299x devices allows a greater
stability with ceramic capacitors combined with ESR and reduces overall system cost.
6.2
Ceramic Capacitors
Ceramic capacitors do contain some parasitic ESR, but for capacitance values greater than 1 uF, the
value of ESR is usually in the range of a few mΩ at high frequencies. . This makes ceramic capacitors
extremely attractive for bypassing high frequency noise and supporting rapidly changing load transients.
But it also makes them unsuitable for use with linear regulator based devices, which were designed to rely
on the output capacitor’s ESR for the loop compensation zero. When using ceramic capacitors at the
output large load steps can cause ringing on VTT as shown in Figure 8.
Figure 8. Ringing at VTT of LP2998 with 100uF Ceramic Capacitor
Although the amount of undershoot was reduced by ceramic capacitor, the absence of sufficient ESR
caused the VTT ringing. Adding an external resistor in series with the VTT capacitor can compensate the
loop and eliminate the ringing. To calculate the value of ESR, first measure the ringing frequency and use
the following equation to calculate the amount of ESR needed for loop compensation.
ESR
1
2SfC VTT
In most cases if a 100uF to 300uF capacitor is used for VTT line, using an ESR in 50mΩ to 100mΩ range
will stabilize the VTT voltage completely as shown in Figure 9.
6
Limiting DDR Termination Regulators’ Inrush Current and Capacitor
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VTT Capacitor Guidelines for LP299x Devices
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Figure 9. Stabilized VTT with 100mΩ ESR in series with 100uF Capacitor
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