Datasheet

V
OUT1
V
OUT2
V
EN
VOLTAGE
TIME
VOLTAGE
TIME
SIMULTANEOUS START UP
RATIOMETRIC START UP
=1R
V
OUT1
V
OUT2
V
EN
OUT12OUT
Vx8.0<V
( )
x
-1
=1R
2R
V
1OUT
2R
x
-1
V
2OUT
V8.0
¸
¹
·
¨
©
§
¸
¨
LM20333
SNVS558D –MAY 2008–REVISED APRIL 2013 www.ti.com
Since the soft-start charging current I
SS
is always present on the SS/TRK pin, the size of R2 should be less than
10 kΩ to minimize the errors in the tracking output. Once a value for R2 is selected the value for R1 can be
calculated using appropriate equation in Figure 31, to give the desired start up. Figure 30 shows two common
start up sequences; the top waveform shows a simultaneous start up while the waveform at the bottom illustrates
a ratiometric start up.
Figure 31. Common Start Up Sequences
A simultaneous start up is preferred when powering most FPGAs, DSPs, or other microprocessors. In these
systems the higher voltage, V
OUT1
, usually powers the I/O, and the lower voltage, V
OUT2
, powers the core. A
simultaneous start up provides a more robust power up for these applications since it avoids turning on any
parasitic conduction paths that may exist between the core and the I/O pins of the processor.
The second most common power on behavior is known as a ratiometric start up. This start up is preferred in
applications where both supplies need to be at the final value at the same time.
Similar to the soft-start function, the fastest start up possible is 1ms regardless of the rise time of the tracking
voltage. When using the track feature the final voltage seen by the SS/TRACK pin should exceed 1V to provide
sufficient overdrive and transient immunity.
BENEFIT OF AN EXTERNAL SCHOTTKY
The LM20333 employs a 40ns dead time between conduction of the control and synchronous FETs in order to
avoid the situation where both FETs simultaneously conduct, causing shoot-through current. During the dead
time, the body diode of the synchronous FET acts as a free-wheeling diode and conducts the inductor current.
The structure of the high voltage DMOS is optimized for high breakdown voltage, but this typically leads to
inefficient body diode conduction due to the reverse recovery charge. The loss associated with the reverse
recovery of the body diode of the synchronous FET manifests itself as a loss proportional to load current and
switching frequency. The additional efficiency loss becomes apparent at higher input voltages and switching
frequencies. One simple solution is to use a small 1A external Schottky diode between SW and GND as shown
in Figure 38. The external Schottky diode effectively conducts all inductor current during the dead time,
minimizing the current passing through the synchronous MOSFET body diode and eliminating reverse recovery
losses.
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