Datasheet

Sync_Dmin
t
1 -
fnom
f
sync
0
50 100 150 200 250 300
R
FREQ
(k:)
100
200
300
400
500
600
SWITCHING FREQUENCY (kHz)
LM26003
SNVS576D –AUGUST 2008–REVISED MARCH 2013 www.ti.com
During high over-current conditions, such as output short circuit, the LM26003 employs frequency foldback as a
second level of protection. If the feedback voltage falls below the short circuit threshold of 0.9V, operating
frequency is reduced, thereby reducing average switch current. This is especially helpful in short circuit
conditions, when inductor current can rise very high during the minimum on-time. Frequency reduction begins at
20% below the nominal frequency setting. The minimum operating frequency in foldback mode is 45 kHz typical.
If the FB voltage falls below the frequency foldback threshold during frequency synchronized operation, the
SYNC function is disabled. Operating frequency versus FB voltage in short circuit conditions is shown in the
Typical Performance Characteristics section.
Under conditions where the on-time is close to minimum (less than 200 ns typically), such as high input voltage
and high switching frequency, the current limit may not function properly. This is because the current limit circuit
cannot reduce the on-time below minimum which prevents entry into frequency foldback mode. There are two
ways to ensure proper current limit and foldback operation under high input voltage conditions. First, the
operating frequency can be reduced to increase the nominal on-time. Second, the inductor value can be
increased to slow the current ramp and reduce the peak over-current.
FREQUENCY ADJUSTMENT AND SYNCHRONIZATION
The switching frequency of the LM26003 can be adjusted between 150 kHz and 500 kHz using a single external
resistor. This resistor is connected from the FREQ pin to ground as shown in the typical application. The resistor
value can be calculated with the following empirically derived equation:
R
FREQ
= (6.25 x 10
10
) x f
SW
-1.042
(7)
Figure 17. Switching Frequency vs R
FREQ
The switching frequency can also be synchronized to an external clock signal using the SYNC pin. The SYNC
pin allows the operating frequency to be varied above and below the nominal frequency setting. The adjustment
range is from 30% above nominal to 20% below nominal. External synchronization requires a 1.23V minimum
(typical) peak signal level at the SYNC pin. The FREQ resistor must always be connected to initialize the nominal
operating frequency. The operating frequency is synchronized to the falling edge of the SYNC input. When
SYNC goes low, the high-side switch turns on. This allows any duty-cycle to be used for the sync signal when
synchronizing to a frequency higher than nominal. When synchronizing to a lower frequency, however, there is a
minimum duty-cycle requirement for the SYNC signal, given in the equation below:
(8)
Where fnom is the nominal switching frequency set by the FREQ resistor, and fsync is a square wave. If the
SYNC pin is not used, it must be pulled low for normal operation. A 10 kΩ pull-down resistor is recommended to
protect against a missing sync signal. Although the LM26003 is designed to operate at up to 500 kHz, maximum
load current may be limited at higher frequencies due to increased temperature rise. See the Thermal
Considerations and TSD section.
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