Technical Reference Note (APC08)
Method 1: External Trim Resistor (continued)
Example
Module version: APC08F03-9 (4.0 to 6.Vin, 3.3Vo).
Requiring to adjust the output voltage from Vo = 3.3V to Vot = 3.3 (1-0.02) = 3.234V.
Vo = 3.3V, Vot = 3.234V, Vref = 0.875V, R2 = 1.13kW (from TABLE 1).
Based on Equation (1), Rt can be determined as 111.9kW.
Trim Function (continued)
Method 2: External DC Source
By connecting an external DC supply across P4 (Enable) and P3 (GND) through a limiting resistor Rt, (see Figure 5), output
voltage adjustment can also be achieved. Equation 3 provides the relationship between the External DC supply, Vt, and Vo
(where Vo is the desired output voltage).
Rt Rt
+
Rt
æ
ö
Equation (3)
Vt = 1+
Vr -
÷
Vo
ç
R1 R2
R1
è
ø
Given: Rt = 10kW
Figure 5. External DC source for output trim adjust.
Vo Adjustment to Lower Voltages. This method does not limit the recommended lower Vo adjustment to 2% as mentioned
on previous sections re: Vo adjustment through external trim resistor.
Example:
Module version: APC08G03-9 (3V to 6Vin, 2.5Vo).
Requiring to adjust the output voltage from Vo = 2.5V to 1.8V
Vo = 1.8V, V = 0.87V, R1 = 3.09kW, R2 = 1.69kW (from Table 1). Based on Equation (3), Vt = 3.0V.
r
Example:
Module version: APC08G03-9 (3V to 6Vin, 2.5Vo).
Requiring to adjust the output voltage from Vo = 2.5V to 0.9V
Vo = 0.9V, V = 0.87V, R1 = 3.09kW, R2 = 1.69kW (from Table 1). Based on Equation (3), Vt = 5.9V.
r
MODEL: APC08 SERIES
SHEET 10 OF 23
OCTOBER 4, 2004 - REVISION 03
ASTEC [ Astec America, Inc ]
