14

3) Thermal Considerations

the droop resistor will increase causing the resistance

to increase. Also, the ambient temperature variation

will contribute to the increase of the resistance,

according to the formula:

where:

α =

T= operating temperature

R = desired droop resistor value

For temperature T = 50˚C,

the % R change = 12%

Droop Resistor Tolerance

Tolerance due to sheet resistivity variation

16%

Tolerance due to L/W error

1%

Tolerance due to temperature variation

12%

Total tolerance for droop resistor

29%

In order to determine the droop resistor value the nominal

voltage drop across it at full load has to be calculated. This

voltage drop has to be such that the output voltage full

load is above the minimum DC tolerance spec.

the DC accuracy spec, the voltage drop developed across

the resistor must be calculated as follows:

=

= 43mV

With the CS5166 DAC accuracy being 1%, the internal

error amplifier’s reference voltage is trimmed so that the

output voltage will be 25mV high at no load. With no load,

there is no DC drop across the resistor, producing an out-

put voltage tracking the error amplifier output voltage,

including the offset. When the full load current is deliv-

ered, a drop of -43mV is developed across the resistor.

Therefore, the regulator output is pre-positioned at 25mV

above the nominal output voltage before a load turn-on.

The total voltage drop due to a load step is ∆V-25mV and

the deviation from the nominal output voltage is 25mV

smaller than it would be if there was no droop resistor.

Similarly at full load the regulator output is pre-positioned

at 18mV below the nominal voltage before a load turn-off.

the total voltage increase due to a load turn-off is ∆V-18mV

and the deviation from the nominal output voltage is

18mV smaller than it would be if there was no droop resis-

tor. This is because the output capacitors are pre-charged

to value that is either 25mV above the nominal output

voltage before a load turn-on or, 18mV below the nominal

output voltage before a load turn-off (see Figure 8).

Obviously, the larger the voltage drop across the droop

resistor (the larger the resistance), the worse the DC and

load regulation, but the better the AC transient response.

Current Limit Setpoint Calculations

The following is the design equation used to set the cur-

rent limit trip point by determining the value of the

embedded PCB trace used as a current sensing element.

The current limit setpoint has to be higher than the normal

full load current. Attention has to be paid to the current

2.796V-2.74V

1.3

0.00393

˚C

Application Information: continued

IFB

ISENSE

Q1

Q2

RFB

+

-

Current Limit Comparator

CS5166

Figure 22: Circuit used to determine the voltage across the droop resistor that will trip the internal current sense comparator.