Datasheet

TPS5103
MULTIPLE MODE SYNCHRONOUS DC/DC CONTROLLER
SLVS240A – SEPTEMBER 1999 – REVISED MAY 2001
23
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
output capacitor RMS current
Where I
O(rms)
is the maximum RMS current in the output capacitor (A), and ∆I is the peak-to-peak inductor ripple
current (A).
Example: ∆I = 2 A, so I
O(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:
I
I(rms)
I
2
o
D (1 D)
1
12
D Iripple
2
Where I
I(rms)
is the input RMS current in the input capacitor (A), I
O
is the output current (A), and D is the duty
cycle. From the equation, it can be seen that the highest input RMS current usually occurs at the lowest input
voltage, so it is the worst case design for the input capacitor ripple current.
Example: I
O
= 5 A; D = 0.36
Then, I
I(rms)
= 3.36 A
softstart
The softstart timing can be adjusted by selecting the soft-start capacitor value. The equation is shown below.
C
(soft)
2 T
(soft)
Where C
(soft)
is the softstart capacitance (µF), T
(soft)
is the start-up time on the softstart terminal (s).
Example: T
(soft)
= 5 ms, so, C
(soft)
= 0.01 µF.
current protection
The current protection in the TPS5103 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:
Rcl
r
ds(on)
I
(trip)
Iind(p-p) 2
0.000015
PWM or HYS mode
Rcl
r
ds(on)
I
(trip)
Iind(p-p) 2
0.000005
SKIP mode
Where, Rcl is the external current limit resistor (R10, R11), r
ds(on)
is the high side MOSFET on-time resistance,
I
(trip)
is the required current limit, and Iind(p-p) is the peak-to-peak output inductor current.
Example: PWM mode or HYS mode
r
ds(on)
= 10 mΩ, I
(trip)
= 5 A, Iind(p-p) = 2 A, so Rcl = 4 kΩ
Example: SKIP mode
r
ds(on)
= 10 mΩ, I
(trip)
= 2 A, Iind(p-p) = 1 A, so Rcl = 5 kΩ