Datasheet

TPS5602
DUAL, FAST, HIGH EFFICIENCY CONTROLLER FOR DSP POWER
SLVS217 – JUNE 1999
12
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
application for DSP power
The design shown in this data sheet is a reference design for a DSP application. An evaluation module (EVM),
TPS5602EVM-121 (SLVP121), is available for customer testing and evaluation. The intent is to allow a
customer to fully evaluate the given design using the plug-in EVM supply shown here. The input voltage for this
EVM is from 4.5 V to 9 V. The outputs are 1.8 V at 4 A and 3.3 V at 3 A. By changing few components this EVM
can be used for different operating specifications such as high-input voltage.
This application provides the following power supply sequence: the core power goes up before the I/O supply,
and if the core power is brought down by abnormal condition, the I/O power will be brought down with it.
To help the customers to design the power supply using the TPS5602, key design procedures are shown below:
switching frequency
With hysteretic control, the switching frequency is a function of the input voltage, the output voltage, the
hysteresis window, the delay of the hysteresis comparator and the driver, the output inductance, the resistance
in the output inductor, the output capacitance, the ESR and ESL in the output capacitor, the output current, and
the turn on resistance of high side and low side MOSFET. It is a very complex equation if everything is included.
To make it more useful to the designers, a simplified equation only considers the most influential factors. The
tolerance of this equation is about 30%:
ƒs
Vout
(
Vin Vout
)
ESR 10 10
7
Td Cout
Vin Vin ESR 10 10
–7
Td 0.007 Lout ESL Vin
Where ƒs is the switching frequency (Hz); Vout is the output voltage (V); Vin is the input voltage (V); Cout is the
output capacitance; ESR is the equivalent series resistance in the output capacitor (Ω); ESL is the equivalent
series inductance in the output capacitor (H); Lout is the output inductance (H); and Td is the output feedback
filter time constant (S).
Example: Vin = 5 V, Vout = 1.8 V, Cout = 680 µF: ESR = 40 mΩ; ESL = 3 nH; Lout = 6 µH; Td = 0.5 µs
Then, the frequency fs = 122 kHz.
output inductor ripple current
The output inductor current ripple can affect not only the efficiency and the inductor saturation, but also the
output capacitor selection. The equation is exhibited below:
Iripple
Vin Vout Iout
Rdson R
L
Lout
D Ts
Where Iripple is the peak-to-peak ripple current through the inductor (A); Vin is the input voltage (V); Vout is the
output voltage (V); Iout is the output current; Rdson is the on-time resistance of MOSFET (Ω); D is the duty cycle;
and Ts is the switching cycle (S). From the equation, it can be seen that the current ripple can be adjusted by
changing the output inductor value.
Example: Vin = 5 V, Vout = 1.8 V, Iout = 5 A: Rdson = 10 mΩ; RL = 5 mΩ; D = 0.36; Ts = 10 mS; Lout = 6 µH
Then, the ripple current Iripple = 2 A.