Datasheet

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SLVS289A − MARCH 2000 − REVISED OCTOBER 2000
23
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
Table 4. LDO Power Stage Components
LDO Section
Ref. Des Part V
IN
V
OUT
Description
Q2:A IRF7811(EVM)
or
Si4410, IRF7413
‡
FDS6680
V
IN
V
IN
− V
DROPOUT
†
Used as a power distribution switch for
LDO output control
Q2:A IRF9410, Si9410
‡
Low cost solution for low LDO output cur-
rent (V
IN
−V
OUT
)*I
OUT
< 1 W
Q2:A IRF7811
‡
Higher current and still surface mount
1 W < (V
IN
−V
OUT
)*I
OUT
) < 2 W
Q2: B IRLZ24N
‡
High output current requiring heat sink.
Low cost but through−hole package.
(V
IN
−V
OUT
)*I
OUT
> 2 W
†
V
DROPOUT
= I
OUT
× RDSON. It should be as small as possible.
‡
Position available on the EVM board
frequency calculation
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 turnon resistance of high-side and low-side MOSFET. It is a very complex equation if everything is included.
To make it more useful to designers, a simplified equation is developed that considers only the most influential
factors. The tolerance of the result for this equation is about 30%:
f
s
+
V
OUT
ǒ
V
IN
* V
OUT
Ǔ
ȧ
ȡ
Ȣ
ESR *
ǒ
250 10
–9
)T
d
Ǔ
C
out
ȧ
ȣ
Ȥ
V
IN
ǒ
V
IN
ESR
ǒ
250 10
–9
) T
d
Ǔ
) V
hys
L
OUT
* ESL V
IN
Ǔ
Where f
s
is the switching frequency (Hz); V
OUT
is the output voltage (V); V
IN
is the input voltage (V); C
OUT
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); L
OUT
is the output inductance (H); T
d
is output feedback RC filter
time constant (S); V
hys
is the hysteresis window (V).
output voltage setpoint calculation
In some applications, the required output voltage is different from the VID reference voltage. In this case,
external voltage divider can be used for the setpoint adjustment. The voltage divider is composed of two
resistors. The equation for the setpoint is:
R
bottom
+
R
top
V
R
V
O
* V
R
Where V
R
is the reference voltage; V
O
is the required output voltage setpoint. V
R
should be lower than V
O
. In
EVM design, the top resistor is R14 for the LDO output, or R10 for ripple regular output; the bottom resistor is
R15 for LDO output, or R12 for ripple regulator output.