XR76117

15A, 22V Synchronous Step-Down COT Regulator

Description

The XR76117 is a synchronous step-down regulator combining the controller, drivers, bootstrap diode and MOSFETs in a single package for point-of-load supplies. The XR76117 has a load current rating of 15A. A wide 5V to 22V input voltage range allows for single supply operation from industry standard 5V, 12V and 19.6V rails.

With a proprietary emulated current mode Constant On-Time (COT) control scheme, the XR76117 provides extremely fast line and load transient response using ceramic output capacitors. It requires no loop compensation, simplifying circuit implementation and reducing overall component count.  The control loop also provides 0.1% load and 0.1% line regulation and maintains constant operating frequency.  A selectable power saving mode, allows the user to operate in Discontinuous Mode (DCM) at light current loads thereby significantly increasing the converter efficiency.

A host of protection features, including overcurrent, over temperature, overvoltage, short-circuit, open feedback detect and UVLO, helps achieve safe operation under abnormal operating conditions.

The XR76117 is available in a RoHS-compliant, green/halogen-free space-saving 5mm x 6mm QFN package.

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Features

  • Controller, drivers, bootstrap diode and MOSFETs
    integrated in one package
  • 15A  step-down regulator
  • Wide 5.0V to 22V input voltage range
    • 3V to 22V PVIN with 5V bias
    • 4.5V to 5.5V single supply operation
    • ≥0.6V adjustable output voltage
  • Proprietary constant on-time control
    • No loop compensation required
    • Stable ceramic output capacitor operation
    • Programmable 70ns to 1μs on-time
    • Constant 200kHz to 1MHz frequency
  • Selectable CCM or CCM/DCM
    • CCM/DCM for high efficiency at light load
    • CCM for constant frequency at light load
  • Precision enable
  • Low impedance power good flag with unpowered
  • Programmable soft-start
  • 17-pin 5mm x 6mm QFN package
  • Pin compatible 20A regulator – XR76121

Application

  • Servers
  • Point-of-Load (POL) converters
  • Industrial control and automation
  • FPGA, DSP and processor supplies
  • Base stations, switches/routers
  • Telecommunications and infrastructure equipment

Design Tools

Packaging

Pkg Code Details Quantities Dimensions PDF
QFN17 5x6
  • JEDEC Reference: MO220
  • MSL Pb-Free: L2 @ 260
  • MSL SnPb Eutectic:
  • ThetaJA: 26.9
  • Bulk Pack Style: Tray
  • Quantity per Bulk Pack: 490
  • Quantity per Reel: 3000
  • Quantity per Tube: N/A
  • Quantity per Tray: 490
  • Reel Size (Dia. x Width x Pitch): 330x12x8
  • Tape & Reel Unit Orientation: Quadrant 1
  • Dimensions: mm
  • Length: 5
  • Width: 6
  • Thickness: 0.90
  • Lead Pitch: 0.65

Parts & Purchasing

Part Number Pkg Code RoHS Min Temp Max Temp Status Buy Now Order Samples
XR76117EL-F QFN17 5x6 -40 125 Active
XR76117ELMTR-F QFN17 5x6 -40 125 OBS Suggested:
XR76117ELTR-F
XR76117ELTR-F QFN17 5x6 -40 125 Active
XR76117EVB Board Active
Show obsolete parts
Part Status Legend
Active - the part is released for sale, standard product.
EOL (End of Life) - the part is no longer being manufactured, there may or may not be inventory still in stock.
CF (Contact Factory) - the part is still active but customers should check with the factory for availability. Longer lead-times may apply.
PRE (Pre-introduction) - the part has not been introduced or the part number is an early version available for sample only.
OBS (Obsolete) - the part is no longer being manufactured and may not be ordered.
NRND (Not Recommended for New Designs) - the part is not recommended for new designs.
Distribution Date Description File
07/04/2018 Product Discontinuation Notice PDN 18-0624-01-1033.pdf
12/19/2016 Qualification of an alternate lead frame supplier, DCI. PCN 16-1146-01-02 DCI lead frame-1033.pdf

Frequently Asked Questions

Voltage mode PWM

Voltage mode PWM is a simple technique that uses a single loop to control the output voltage. As shown in Figure 1, the output voltage is compared to a reference voltage with an error amplifier. The output of the error amplifier is then compared to a sawtooth and that output is used to drive the MOSFET, usually via a voltage divider.

 


Figure 1

 

As shown in Figure 2, the output voltage is modulated by turning the high-side FET on (on-time) with the pulse width and turning the low side FET off. At the end of the pulse, the high-side FET is turned off (off-time) and the low side FET is turned on until the next pulse. Vout = On-Time/Period * Vin.

 

Figure 2
 

The advantages of voltage mode PWM is that it is a very simple, common, smaller solution with good accuracy. The disadvantages are that complex frequency compensation is required (two poles) to stabilize the loop and because trailing edge control is most commonly used, there is a delay in load step response.

Current mode PWM

With voltage mode PWM, current is less known. For better control, current mode PWM senses the inductor current and it is compared to the reference voltage as shown in Figure 3.

 


Figure 3

 

Although the current has to be sensed with accuracy and introduces noise, the advantages of current mode PWM are easier loop compensation (less compensation needed with one pole), and it is easier to implement over-current protection and parallel currents to the output.

Standard Constant On-Time (COT)

As opposed to PWM, the pulse width in COT is always the same as shown in Figure 4. Instead the off-time length varies (as does the frequency) which modulates the output. As the Vout increases, the off-time of the duty cycle increases (frequency decreases) and the fixed on-time produces a lower duty cycle. This transfers less energy to the output and lowers the Vout. More simply said, as Vout increases, the duty cycle decreases. Conversely, as the Vout decreases, the off-time of the duty cycle decreases (frequency increases) and the fixed on-time produces a higher duty cycle. This transfers more energy to the output and raises the Vout.

 


Figure 4

 

The advantages of standard COT are very fast transient response, simplicity (inexpensive) and that frequency compensation is not complex as it is in PWM control. However, the feedback signal tends to have low amplitude and signal to noise ratio, making it very noise sensitive. Also, the output voltage is higher than the reference voltage and the ripple is dependent on and sensitive to the output capacitor ESR. This introduces a DC offset which is the average amount the output voltage is over the reference voltage. It is also jitter prone and the frequency changes during the load steps.

Some solutions solve the noise sensitivity by having one of two options that condition the feedback signal but introduce delays. One tradeoff provides faster transient responses; the other allows low ESR output capacitors to be used.

MaxLinear’s patented COT

MaxLinear’s patented COT architecture however conditions the reference instead as shown in Figure 5. The MaxLinear devices create their own emulated ramp that is insensitive to noise and the ESR of the capacitor. Since the output capacitor ESR does not affect it, low ESR ceramic capacitors can be used and maintain stability without decreasing speed. In addition, the Vout and reference voltage are compared and that result controls the ramp circuit. This creates a slower loop where the output voltage is averaged out and the DC offset is not introduced as in standard COT.

 


Figure 5

 

MaxLinear’s COT still has the standard COT advantages of very fast transient response, simplicity and no complex frequency compensation in addition to not having DC offset or ESR value sensitivity. MaxLinear’s COT architecture provides exceptional line and load regulation.

The XR76117 and XR76121 are identical to each other except for their output current rating. Similarly the IR3824/IR3825/IR3829 are identical to each other except for the differences shown in this section. Output current ratings are summarized in Table 1.

  

Spec

Exar XR76117 / XR76121

Infineon IR3824 / IR3825 / IR3829

Max IOUT

15A (XR76117); 20A (XR76121)

15A (IR3824); 16A (IR3829); 20A (IR3825)

VIN range

4.5V to 22V

5V to 21V

VOUT range

0.6V to 18V

0.6V to 0.86 x VIN

Frequency range

200kHz to 1MHz

Up to 1.5MHz

Temperature range

-40°C to 125°C

-40°C to 125°C

Supply current

17mA

20mA


Table 1: Major Specification Comparison 

 
The XR76117 / XR76121 can be soldered into a IR3824/25/29 socket, only minor board stuff options are required. The following and Application Note ANP-49 discuss how both series can occupy the same socket on a PCB.

 

Pin-out comparison

 

 
                                                                                 
Figure 1: Pin-out Comparison 
 

Pin

XR76117 / XR76121

IR3824 / IR3825 / IR3829

Same

Function /

Connection?

Changes Required to Drop XR76117/21 into IR3824/25/29 Socket

1

FB

Fb

Yes/Yes

None

2

FCCM

NC

No/No

Must either be tied to VCC for FCCM operation or tied to GND for CCM/DCM operation. Pull-up or pull-down resistors may also be used. See Figure 2 and XR76117 or XR761121 datasheet for more information.

3

AGND

Comp

No/No

Add a 0Ω resistor to jumper pin 3 to pin 4 and do not stuff compensation Cs/Rs. Refer to Figure 2.

4

AGND

Gnd

Yes/Yes

None

5

TON

Rt

No/Yes

In both cases, this pin sets frequency and requires a resistor to GND. However, a different resistor value must be used for the XR solution. Refer to the XR76117 / XR76121 datasheet for resistor value calculation.

6

ILIM

ILIM

Yes/No

Add a 0Ω resistor to connect to SW. Do not stuff resistors to VCC or GND. Refer to Figure 2.

7

PGOOD

PGood

Yes/Yes

None

8

VSNS

Vsns

Yes/Yes

None

9

VIN

Vin

Yes/Yes

None

10

VCC

Vcc/

LDO_Out

Yes/Yes

None

11

PGND

PGnd

Yes/Yes

None

12

SW

SW

Yes/Yes

None

13

PVIN

PVin

Yes/Yes

None

14

BST

Boot

Yes/Yes

None

15

EN

Enable

Yes/Yes

None

16

SS

NC

No/No

Add an external capacitor to GND. See Figure 2 or the XR76117/XR76121 datasheet for more information.

17

AGND PAD

GND

Yes/Yes

None


Table 2: Pin-out Comparison & Changes Required to Drop XR76117 / XR76121 into IR3824 / IR3825 / IR3829 Socket 

 
Board Stuff Option Schematic
 

The PCB board can be easily designed to drop-in the XR76117 or XR76121 while maintaining compatibility to the IR3824/25/29. In Figure 2 below, pinning for both series are represented. The Exar XR76117 and XR76121 pin names do not have parenthesis, and the same corresponding pins for the IR3824/25/29 are in parenthesis. As shown in the legend, the components in green boxes are added and the ones in red boxes are omitted when using the XR76117 or XR76121. Both a pull-up and pull-down are shown for pins 2 and 6, but only one or the other will be present depending on the application. So, 4 passives (red) will not be populated while 3 passives and a jumper will be populated.

 
 


Figure 2: Addition and Omission of External Components 
 

Find the product page of the part that you want to get an evaluation board for and click on Parts & Purchasing. Example:

 

Find the icons under Buy Now or Order Samples:

 
 

Click on the Buy Now icon and see who has stock and click on the Buy button:

 
 
 

Alternatively, you can click on the Order Samples

 
 

If the icons are missing, then contact Customer Support.

In PWM controllers, frequency is constant and tON (on-time) is set by the controller to regulate the output voltage. However in COT controllers, the tON is constant and set by the RON resistor. This also sets the frequency. RON is connected between the TON pin and GND.

If the high-side FET was ideal, the tON of the SW signal would be equal to tON of GH (high-side FET gate) signal and the relationship between RON and tON would be:

 
 

However, the high side FET has rise and fall times as well as on and off delay times, so the tON of SW is not equal to GH. This non-ideal characteristic is measured for each regulator or module where the above equation is modified. In the Applications Information of each regulator or module datasheet, an equation defining the relationship of RON and tON is given based on test data for that device. For example, in the XR79206 power module datasheet, you will find the following equation in the Programming the On-Time section of the Applications Information:

 
 
 

The correlation of this equation to the test data is also given in the datasheet. In the XR79206 example, Figure 5 in the Typical Performance Characteristics section shows very good correlation:

 
 
 In an ideal Buck Converter, tON is a function of VIN, VOUT and f expressed in following equation:
 
 

However, as no Buck Converter is ideal, test data is taken to determine a more accurate equation which is also given in the datasheet. In the XR79206 example, the following equation is given based on test data:

 
 
 

Substituting this tON equation into the above equation relating RON and tON and simplifying, we get:

 
 
 

Then RON can be easily chosen based on the targeted VIN, VOUT, frequency and efficiency. So for example in the XR79206, if VIN = 24V, VOUT = 5V, f = 500kHz and efficiency = 90%,

 
 
 
 
The next closest commercially available resistor value can be used. Several RON examples are given in the datasheets based on the above equation for your convenience. For a given RON it should be noted that tON is inversely proportional to VIN. This inverse relationship allows the frequency to remain constant as VIN changes, except for some changes due to the non-ideal nature of the power components. For example, this is illustrated in the following graph from the XR79206:
 
 
 

As IOUT increases, frequency increases slightly due to increasing power losses. As losses increase, more power must be delivered per cycle to keep VOUT constant. Because tON is constant, the period decreases and frequency increases, as can be seen in the following example from the XR79206 datasheet:

 
 
 
 
 
 
The best way to determine this is to go to exar.com and type the part into the search function. At or near the top of the results you should see something that looks like
 
 

In this example, we looked for XRA1201. When you hover over it, it will turn grey and you can click anywhere in the grey box. This brings you to the product page. For example:

 
 

Click on Parts & Purchasing, highlighted in yellow above. The screen changes to:

 

Notice the status column and the “Show obsolete parts” link:

 

A legend tells you the definition of the different statuses. Click on the “Show obsolete parts” link to see EOL or OBS part numbers along with the Active part numbers:

 
 

Another method to find out if a part is OBS or EOL is to click on SUPPORT:

 

And then Product Change Notifications

 
 

Type the part into the search, and click on one of the part numbers from the drop down menu. Then you can look for the Product Discontinuation Notice, which generally is at the top of the list, for example:

 
 

If you see this, it tells you that this particular orderable part has been discontinued and when the last order date is, or was. If you click on the file, then you can view the notice we sent about this if you purchased the part in the recent past. It may also advise of a replacement part. When an orderable part first becomes discontinued, Product Discontinuation Notices are sent are sent to those who have purchased the parts in the recent past, if purchased directly, with a dated opportunity to place a last order.

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