Data Sheet

LTM4640

analog.com

Rev. 0

20 of 38

Since the subordinate regulator’s TRACK/SS is connected to the main device output through an RTR(TOP)/RTR(BOT)

resistor divider, and its voltage is used to regulate the subordinate device output voltage when TRACK/SS voltage is

below 0.6V, the subordinate device output voltage and the main device output voltage should satisfy Equation 7

during startup.

������������������������(������������������) ×

������������������(������������������)

������������������(������������������) + 10������Ω

= ������������������������(������������������������) ×

������������������(������������������)

������������������(������������������) + ������������������(������������������)

(7)

The RFB(SUB)is the feedback resistor, and the RTR(TOP)/RTR(BOT)is the resistor divider on the TRACK/SS pin of the

subordinate regulator, as shown in Figure 22.

Following Equation 7, the ratio of the main device output slew rate (MR) to the subordinate device output slew

rate (SR) is given by Equation 8.

������������

������������

=

������������������(������������������)

������������������(������������������) + 10������Ω

������������������(������������������)

������������������(������������������) + ������������������(������������������)

(8)

For example, VOUT(MAIN)= 1.5V, MR = 1.5V/1ms and VOUT(SUB)= 1.2V, SR = 1.2V/1ms. From Equation 8, we could solve that

RTR(TOP)= 10kΩ and RTR(BOT)= 6.65kΩ are a good combination for the ratio metric tracking.

The TRACK/SS pin has the 6µA current source on when a resistive divider implements tracking on the subordinate

regulator. This imposes an offset on the TRACK/SS pin input. Smaller value resistors with the same ratios as the

resistor values calculated from Equation 8 are recommended to mitigate such impact. For example, where the

10kΩ is used, then a 5kΩ is used to reduce the TRACK/SS pin offset to a negligible value.

Coincident output tracking is recognized as a special ratio metric output tracking in which the main device output

slew rate (MR) is the same as the subordinate device output slew rate (SR), waveform as shown in Figure 23.

Figure 23. Output Coincident Tracking Waveform

TIME

SUBORDINATE OUTPUT

MAIN OUTPUT