Datasheet

+
BOOT
HG
LG
V
IN
V
O
V
CC
+
LM2745/8
D1
D2D3
LM2745, LM2748
SNOSAL2E –APRIL 2005–REVISED APRIL 2013 www.ti.com
Figure 27. Charge Pump with Added Gate Drive
All the gate drive circuits shown in the above figures typically use 100 nF ceramic capacitors in the bootstrap
locations.
POWER GOOD SIGNAL
The open drain output on the Power Good pin needs a pull-up resistor to a low voltage source. The pull-up
resistor should be chosen so that the current going into the Power Good pin is less than 1 mA. A 100 kΩ resistor
is recommended for most applications.
The Power Good signal is an OR-gated flag which takes into account both output overvoltage and undervoltage
conditions. If the feedback pin (FB) voltage is 18% above its nominal value (118% x V
FB
= 0.708V) or falls 28%
below that value (72% x V
FB
= 0.42V) the Power Good flag goes low. The Power Good flag can be used to signal
other circuits that the output voltage has fallen out of regulation, however the switching of the LM2745/8
continues regardless of the state of the Power Good signal. The Power Good flag will return to logic high
whenever the feedback pin voltage is between 72% and 118% of 0.6V.
UVLO
The 2.79V turn-on threshold on V
CC
has a built in hysteresis of about 300 mV. If V
CC
drops below 2.42V, the chip
definitely enters UVLO mode. UVLO consists of turning off the top and bottom MOSFETS and remaining in that
condition until V
CC
rises above 2.79V. As with normal shutdown initiated by the SD pin, the soft-start capacitor is
discharged through an internal MOSFET, ensuring that the next start-up will be controlled by the soft-start
circuitry.
CURRENT LIMIT
Current limit is realized by sensing the voltage across the low-side MOSFET while it is on. The R
DSON
of the
MOSFET is a known value; hence the current through the MOSFET can be determined as:
V
DS
= I
OUT
x R
DSON
The current through the low-side MOSFET while it is on is also the falling portion of the inductor current. The
current limit threshold is determined by an external resistor, R
CS
, connected between the switching node and the
I
SEN
pin. A constant current (I
SEN-TH
) of 40 µA typical is forced through R
CS
, causing a fixed voltage drop. This
fixed voltage is compared against V
DS
and if the latter is higher, the current limit of the chip has been reached.
To obtain a more accurate value for R
CS
you must consider the operating values of R
DSON
and I
SEN-TH
at their
operating temperatures in your application and the effect of slight parameter differences from part to part. R
CS
can be found by using the following equation using the R
DSON
value of the low side MOSFET at it's expected hot
temperature and the absolute minimum value expected over the full temperature range for the for the I
SEN-TH
which is 25 µA:
R
CS
= R
DSON-HOT
x I
LIM
/ I
SEN-TH
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