ABR-42019| Application Note

August 1999

Application Brief 42019 Calculating Frequency Setting Resistors when Using the ML4835
INTRODUCTION
This Application Brief describes a procedure that can be used to calculate the value of resistors used to set preheating, starting, and full power operating frequencies. In this example the inverter frequencies used for preheating, starting and full power are 64.5kHz, 50kHz and 40kHz, respectively.

SOLVE FOR RSET
RSET is the resistor that sets the preheating frequency. To simplify the solution for ICHG let V1 = VREF VTL and let V2 = VREF VTH. Then substitute into and reconfigure equation 4 from page 10

SOLVE FOR RT
RT is the resistor that sets the minimum or full power operating frequency. The following values are taken from the ML4835 Data Sheet: VREF of 7.5V and IDIS of 5.5mA (Electrical Characteristics, page 4); and VTH of 3.75V and VTL of 1.25V (Figure 9, page 9). To calculate RT a value for CT must first be chosen. A value of 1.5nF for CT produces a reasonable discharge or dead time (see Electrical Characteristics, page 4) and keeps ICHG <0 after the preheating time interval (see note 1 in Figure 9 on page 9).
t DIS =

ICHG

(e ) = ) 1 R (e
V1 V2
T t CHG RT C T

t CHG RTC T

From equation 3 on page 10
t CHG = 1 t DIS = 7us fOSC

Then ICHG = 190uA. From equation 6 on page 10
ICHG = 2.5 R SET , so RSET = 13.1k

VTL C T IDIS

= 0.68us

This value of RSET satisfies the requirement of note 1 in figure 9 on page 9.

RT can then be solved. For the case where fOSC = 2 times the inverter frequency, or 80kHz:
t DIS fOSC = 14.5k. RT = 0.544C T 1

SOLVE FOR RT2
The parallel equivalent resistance of RT and RT2 determines the starting frequency. From equation 5a on page 10
R T R T2 = 1 0.544 fOSC C T = 12.25k

So RT2 = 79.1k.

REV. 1.0 10/25/2000