TPS5210
PROGRAMMABLE SYNCHRONOUS BUCK REGULATOR CONTROLLER
SLVS171A – SEPTEMBER 1998 – REVISED MAY 1999
21
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
APPLICATION INFORMATION
frequency calculation
A detailed derivation of frequency calculation is shown in the application report, “Designing Fast Response
Synchronous Buck Regulators Using the TPS5210”, TI Literature number SLVA044. When less accurate results
are acceptable, the simplified equation shown below can be used:
f
s
V
O
V
I
V
O
ESR
V
I
L
Hysteresis Window
Control Section
Below are the equations needed to select the various components within the control section. Details and the
derivations of the equations used in this section are available in the application report “Designing Fast Response
Synchronous Buck Regulators Using the TPS5210”, TI Literature number SLVA044.
output voltage selection
Of course the most important function of the power supply is to regulate the output voltage to a specific value.
Values between 1.3 V and 3.5 V can be easily set by shorting the correct VID inputs to ground. Values above
the maximum reference voltage (3.5 V) can be set by setting the reference voltage to any convenient voltage
within its range and selecting values for R2 and R3 to give the correct output. Select R3:
R3 << than V
REF
/I
BIAS(VSENSE)
; a recommended value is 10 k
Then, calculate R2 using:
V
O
V
REF
1
R2
R3
R2
R3
V
O
V
REF
V
REF
or
R2 and R3 can also be used to make small adjusts to the output voltage within the reference-voltage range
and/or to adjust for load-current active droop compensation. If there is no need to adjust the output voltage, R3
can be eliminated. R2, R3 (if used), and C7 are used as a noise filter; calculate using:
C7
150 ns
R2
R3
slowstart timing
Slowstart reduces the startup stresses on the power-stage components and reduces the input current surge.
Slowstart timing is a function of the reference-voltage current (determined by R6) and is independent of the
reference voltage. The first step in setting slowstart timing will be to determine R6:
R6 should be between 7 k
and 300 k
, a recommended value is 20 k
.
Set the slowstart timing using the formula:
C5
t
SS
R
VREFB
5
t
SS
(5
R6)
Where C5
= Slowstart capacitance in
μ
F
t
SS
= Slowstart timing in
μ
s
R
VREFB
= Resistance from VREFB to GND in ohms (
≈
R6)
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