Datasheet

R
ON
=
V
OUT
x (V
IN
- 1.4V)
V
IN
x F
S
x 1.18 x 10
-10
- 1.4 k:
R
ON
=
(t
ON
- 67 ns) x (V
IN
- 1.4V)
1.18 x 10
-10
- 1.4 k:
t
ON
=
1.18 x 10
-10
x (R
ON
+ 1.4k)
(V
IN
- 1.4V)
+ 67 ns
FB
SW
L1
C2
R1
R2
R3
BST
VCC
D2
C3
C4
D1
LM5010A
VOUT
SGND
ISEN
LM5010A
LM5010A-Q1
www.ti.com SNVS376E –OCTOBER 2005–REVISED FEBRUARY 2013
The turn-on sequence is shown in Figure 7. When VCC exceeds the under-voltage lock-out threshold (UVLO) of
5.25V (t1 in Figure 7), the buck switch is enabled, and the SS pin is released to allow the soft-start capacitor (C6)
to charge up. The output voltage V
OUT
is regulated at a reduced level which increases to the desired value as the
soft-start voltage increases (t2 in Figure 7).
Figure 8. Self Biased Configuration
Regulation Comparator
The feedback voltage at the FB pin is compared to the voltage at the SS pin (2.5V, ±2%). In normal operation an
on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch conducts for the on-time
programmed by R
ON
, causing the FB voltage to rise above 2.5V. After the on-time period the buck switch
remains off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 5 nA over
temperature.
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the
on-time is immediately terminated. This condition can occur if the input voltage, or the output load, changes
suddenly. The buck switch remains off until the voltage at FB falls below 2.5V.
ON-Time Control
The on-time of the internal buck switch is determined by the R
ON
resistor and the input voltage (V
IN
), and is
calculated as follows:
(5)
The R
ON
resistor can be determined from the desired on-time by re-arranging Equation 5 to the following:
(6)
To set a specific continuous conduction mode switching frequency (f
S
), the R
ON
resistor is determined from the
following:
(7)
In high frequency applications the minimum value for t
ON
is limited by the maximum duty cycle required for
regulation and the minimum off-time of the LM5010A (260 ns, ±15%). The fixed off-time limits the maximum duty
cycle achievable with a low voltage at VIN. The minimum allowed on-time to regulate the desired V
OUT
at the
minimum V
IN
is determined from the following:
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