LM20333EVAL National Semiconductor, LM20333EVAL Datasheet - Page 15

• Manufacturer Part Number: LM20333EVAL
• Description: EVALUATION BOARD FOR THE LM20333
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM20333EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 4.5 ~ 25V
• Regulator Topology: Buck
• Board Type: Fully Populated
• Utilized Ic / Part: LM20333
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

A simultaneous start up is preferred when powering most FP-
GAs, DSPs, or other microprocessors. In these systems the
higher voltage, V
voltage, V
vides a more robust power up for these applications since it
avoids turning on any parasitic conduction paths that may ex-
ist 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 volt-
age 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 re-
verse 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 14. The ex-
ternal Schottky diode effectively conducts all inductor current
during the dead time, minimizing the current passing through
FIGURE 7. Common Start Up Sequences
OUT2
, powers the core. A simultaneous start up pro-
OUT1
, usually powers the I/O, and the lower
30051678
15
the synchronous MOSFET body diode and eliminating re-
verse recovery losses.
The external Schottky conducts currents for a very small por-
tion of the switching cycle, therefore the average current is
low. An external Schottky rated for 1A will improve efficiency
by several percent in some applications. A Schottky rated at
a higher current will not significantly improve efficiency and
may be worse due to the increased reverse capacitance. The
forward voltage of the synchronous MOSFET body diode is
approximately 700 mV, therefore an external Schottky with a
forward voltage less than or equal to 700 mV should be se-
lected to ensure the majority of the dead time current is carried
by the Schottky.
THERMAL CONSIDERATIONS
The thermal characteristics of the LM20333 are specified us-
ing the parameter θ
to the ambient temperature. Although the value of θ
pendant on many variables, it still can be used to approximate
the operating junction temperature of the device.
To obtain an estimate of the device junction temperature, one
may use the following relationship:
and
Where:
T
P
θ
LM20333.
T
I
DCR is the inductor series resistance.
It is important to always keep the operating junction temper-
ature (T
temperature exceeds 170°C the device will cycle in and out
of thermal shutdown. If thermal shutdown occurs it is a sign
of inadequate heatsinking or excessive power dissipation in
the device.
Figure 8, Figure 9, Figure 10 and Figure 11 can be used as a
guide to avoid exceeding the maximum junction temperature
of 125°C provided an external 1A Schottky diode, such as
Central Semiconductor's CMMSH1-40-NST, is used to im-
prove reverse recovery losses.
OUT
J
JA
A
IN
is the junction temperature in °C.
is the ambient temperature in °C.
is the input power in Watts (P
is the junction to ambient thermal resistance for the
is the output load current.
P
J
D
) below 125°C for reliable operation. If the junction
= P
IN
x (1 - Efficiency) - 1.1 x (I
JA
T
, which relates the junction temperature
J
= P
D
x θ
JA
IN
+ T
= V
A
IN
OUT
x I
IN
)
2
).
x DCR
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JA
is de-