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, V
ref
= 0.875V, R
2
= 1.13kΩ (from TABLE 1).
Based on Equation (1), Rt can be determined as 111.9kΩ.
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
Vt
=
1
+
+
Vr
−
Vo
R
1
R
1
R
2
Equation (3)
Given:
Rt
= 10kΩ
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
r
= 0.87V, R
1
= 3.09kΩ, R
2
= 1.69kΩ (from Table 1). Based on Equation (3), Vt = 3.0V.
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
r
= 0.87V, R
1
= 3.09kΩ, R
2
= 1.69kΩ (from Table 1). Based on Equation (3), Vt = 5.9V.
MODEL: APC08 SERIES
OCTOBER 4, 2004 - REVISION 03
SHEET 10 OF 23
ASTEC [ Astec America, Inc ]
