|Description||6/4-Bit VID Programmable Current DAC for Point of Load Regulators with Adjustable Start-Up Current|
|LM10011SDX/NOPB Datasheet PDF : 24 Pages |
SNVS822A – DECEMBER 2012 – REVISED NOVEMBER 2014
Typical Application (continued)
8.2.1 Design Requirements
Table 4 lists the design parameters.
Table 4. Design Parameters
Output voltage range
0.7 V to 1.1 V
8.2.2 Detailed Design Procedure
184.108.40.206 Setting the VOUT Range and LSB
Looking at the Typical Application Circuit in Figure 12, the following equation defines VOUT of a given regulator
(valid for VOUT > VFB):
- IDAC_OUT x RFB1
Here, the output voltage is a function of the resistor divider from RFB1 and RFB2. Using the LM10011, there is a
current supplied by the IDAC_OUT pin that helps drive current through the feedback resistor RFB2, thus lowering
the necessary current supplied through RFB1, and hence lowering VOUT. To calculate the nominal (maximum)
VOUT, use an IDAC_OUT value of 0 µA.
The change in the output voltage can be analyzed based on the resolution of the current DAC from the LM10011
compared to the desired resolution of the output swing of the regulator. RFB1 is designed to provide the desired
VOUT_LSB with the equation:
VOUT_LSB = LSB x RFB1
Where LSB = LSB_6 (940 nA) from the electrical characteristics table (see Electrical Characteristics). Based on
the desired nominal VOUT (with IDAC_OUT = 0 µA) and the calculated RFB1 from Equation 2, RFB2 can be solved
using Equation 1.
220.127.116.11 4-Bit Mode Design Example
Designing with the LM10011 in 4-bit mode is similar to designing in 6-bit mode. The only differences are the LSB
value (LSB = LSB_4 = 3.76 µA) in Equation 2 and full-scale current range (IDAC_OUT = 56.4 µA).
18.104.22.168 Setting the Start-Up Voltage with RSET
RSET is chosen depending on the required start-up voltage for the particular application. The user must use
Equation 3 and solve for the required IDAC_OUT by inputting the known values of RFB1 and RFB2, VFB, and the
desired start-up output voltage, VOUT. Once IDAC_OUT is solved for, choose an RSET based on Table 2 to select
a start-up code to yield a current closely matching the calculated result. Use the equation below to solve for the
required IDAC_OUT value at start-up.
IDAC_OUT = RFB1
22.214.171.124 Example Solution
While in 6-bit mode, assuming a 400-mV output range, 64 VID codes, and an IDAC LSB of 0.940 µA, it is desired
to have a VOUT with an LSB of 6.4 mV and a default value of 1.1 V with a 1.05-V start-up voltage using an
LM21215A-1 regulator (VFB = 0.6 V):
6.4mV = 0.940µA x RFB1
RFB1 = 6.8k
Using 1% standard resistor values, RFB1 can be set to 6.81 kΩ. Now calculate RFB2 based on RFB1 and the
maximum VOUT of 1.1 V using Equation 1.
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