Datasheet

TPS5602
DUAL, FAST, HIGH EFFICIENCY CONTROLLER FOR DSP POWER
SLVS217 – JUNE 1999
13
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
application for DSP power (continued)
output capacitor RMS current
Assuming the inductor ripple current totally goes through the output capacitor to ground, the RMS current in the
output capacitor can be calculated as:
Io(rms)
I
12
Where Io(rms) is the maximum RMS current in the output capacitor (A); ∆I is the peak–to–peak inductor ripple
current (A).
Example: ∆I = 2 A, so Io(rms) = 0.58 A
input capacitor RMS current
Assuming the input ripple current totally goes into the input capacitor to the power ground, the RMS current in
the input capacitor can be calculated as:
Ii(rms) Io
D
(
1 D
) (
1 D
)
D
Where Ii(rms) is the input RMS current in the input capacitor (A); Io is the output current (A); D is the duty cycle.
From the equation, it can be seen that the highest input RMS current usually occurs at lowest input voltage.
Example: Io = 5 A; D = 0.36
Then, Ii(rms) = 3.36 A
softstart
The softstart timing can be adjusted by selecting the softstart capacitor value. The equation is
C
soft
= 2 × T
soft
Where C
soft
is the softstart capacitance (µF); T
soft
is the start-up time pin (S).
Example: Tsoft = 5 ms, so Csoft = 0.01 µF.
current protection
The current protection in TPS5602 is set using an internal current source and an external resistor to set up the
current limit. The sensed high side MOSFET drain-to-source voltage drop is compared to the set point; if the
voltage drop exceeds the limit, the internal oscillator is activated, and continuously resets the current limit until
the over-current condition is removed. The equation below should be used for calculating the external resistor
value for current protection:
Rclmt
Rds(on) (Itrip Iind(p-p) 2)
0.000015
Where Rclmt is the external current limit resistor (R10, R11); Rds(on) is the high side MOSFET on resistance;
Itrip is the required current limit; lind(p-p) is the peak-to-peak output inductor current.
Example: Rds(on) = 10 mΩ, Itrip = 5 A, Lind = 2 A, so Rclmt = 4 kΩ.