Datasheet

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
V
CC
D2
C3
C4
D1
LM5010
V
OUT2
V
OUT1
S
GND
I
SEN
LM5010
www.ti.com SNVS307F –SEPTEMBER 2004–REVISED FEBRUARY 2013
Start-up Regulator (V
CC
)
The startup regulator is integral to the LM5010. The input pin (V
IN
) can be connected directly to line voltages up
to 75V. The V
CC
output is regulated at 7.0V, ±6%, and is current limited to 10 mA. Upon power up the regulator
sources current into the external capacitor at V
CC
(C3). With a 0.1 µF capacitor at V
CC
, approximately 58 µs are
required for the V
CC
voltage to reach the under-voltage lockout threshold (UVLO) of 5.8V (t1 in Figure 8), at
which time the buck switch is enabled, and the soft start pin is released to allow the soft start capacitor (C6) to
charge up. V
OUT
then increases to its regulated value as the soft start voltage increases (t2 in Figure 8).
The minimum input operating voltage is determined by the regulator’s dropout voltage, the V
CC
UVLO falling
threshold (≊5.65V), and the frequency. When V
CC
falls below the falling threshold the V
CC
UVLO activates to shut
off the buck switch and ground the soft start pin. If V
CC
is externally loaded, the minimum input voltage increases
since the output impedance at V
CC
is ≊140Ω at low V
IN
. See Figure 2 and Figure 3. In applications involving a
high value for V
IN
where power dissipation in the startup regulator is a concern, an auxiliary voltage can be diode
connected to the V
CC
pin (Figure 10). Setting the auxiliary voltage to between 7.5V and 14V shuts off the internal
regulator, reducing internal power dissipation. The current required into the V
CC
pin is shown in Figure 4.
Internally a diode connects V
CC
to V
IN
.
Figure 10. Self Biased Configuration
Regulation Comparator
The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V, ±2%). In normal operation (the
output voltage is regulated) an on-time period is initiated when the voltage at FB falls below 2.5V. The buck
switch stays on for the on-time causing the FB voltage to rise above 2.5V. After the on-time period the buck
switch stays off until the FB voltage falls below 2.5V. 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, change
suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5V.
ON-Time Control
The on-time of the internal switch (see Figure 5) is determined by the R
ON
resistor and the input voltage (V
IN
),
calculated from the following:
(5)
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