Datasheet

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
LM25010
VOUT
SGND
ISEN
LM25010
www.ti.com SNVS419D –DECEMBER 2005–REVISED FEBRUARY 2013
When V
IN
exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 to 3 µs. The
capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its
absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to
the LM25010 pins.
In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An
auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin (D2 in Figure 8) to shut off the
VCC regulator, reducing internal power dissipation. The current required into the VCC pin is shown in the Typical
Performance Characteristics. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less
than V
IN
.
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 (Fs), the R
ON
resistor is determined from the
following:
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