Datasheet

V
OUT
VIN x f
SW
x C
RAMP
25 PA x
I
PEAK
=
A x R
S
V
OUT
VIN x f
SW
x C
RAMP
1.2V - 25 PA x
0.12V
R
S
or
I
PEAK
#
LM25088
LM25088-Q1
SNVS609H –DECEMBER 2008–REVISED MARCH 2013 www.ti.com
where
• A = 10V/V is the current sense amplifier gain
• C
RAMP
is the Ramp capacitor
• R
S
is the sense resistor
• is the voltage ramp added for slope compensation
• 1.2V is the reference of the current limit comparator (7)
Since the current that charges the RAMP capacitor is proportional to VIN-VOUT, if the output is suddenly
shorted, the VOUT term is zero and the RAMP charging current increases. The increased RAMP charging
current will immediately reduce the PWM duty cycle.The LM25088 also includes a buck switch protection
scheme. A dedicated comparator monitors the drain to source voltage of the buck FET when it is turned ON, if
the V
DS
exceeds 1.5V, the comparator turns of the buck FET immediately. This feature will help protect the buck
FET in catastrophic conditions such as a sudden saturation of the inductor.
Overload Protection Timer (LM25088-2 Only)
To further protect the external circuitry during a prolonged over current condition, the LM25088-2 provides a
current limit timer to disable the switching regulator and provide a delay before restarting (hiccup mode). The
number of current limit events required to trigger the restart mode is programmed by an external capacitor at the
RES pin. During each PWM cycle, as shown in Figure 22, the LM25088 either sinks current from or sources
current into the RES capacitor. If the emulated current ramp exceeds the 1.2V current limit threshold, the present
PWM cycle is terminated and the LM25088 sources 50 µA into the RES pin capacitor during the next PWM clock
cycle. If a current limit event is not detected in a given PWM cycle, the LM25088 disables the 50 µA source
current and sinks 27 µA from the RES pin capacitor during the next cycle. In an overload condition, the LM25088
protects the converter with cycle-by-cycle current limiting until the voltage at RES pin reaches 1.2V. When RES
reaches 1.2V, a hiccup mode sequence is initiated as follows:
• The SS capacitor is fully discharged.
• The RES capacitor is discharged with 1.2 µA
• Once the RES capacitor reaches 0.2V, a normal soft-start sequence begins. This provides a time delay
before restart.
• If the overload condition persists after restart, the cycle repeats.
• If the overload condition no longer exists after restart, the RES pin is held at ground by the 27 µA discharge
current source and normal operation resumes.
The overload protection timer is very versatile and can be configured for the following modes of protection:
1. 1. Cycle-by-Cycle only: The hiccup mode can be completely disabled by connecting the RES pin to GND.
In this configuration, the cycle-by-cycle protection will limit the output current indefinitely and no hiccup
sequence will occur.
2. 2. Delayed Hiccup: Connecting a capacitor to the RES pin provides a programmed number of cycle-by-
cycle current limit events before initiating a hiccup mode restart, as previously described. The advantage of
this configuration is that a short term overload will not cause a hiccup mode restart but during extended
overload conditions, the average dissipation of the power converter will be very low.
3. 3. Externally Controlled Hiccup: The RES pin can also be used as an input. By externally driving the pin to
a level greater than the 1.2V hiccup threshold, the controller will be forced into the delayed restart sequence.
For example, the external trigger for a delayed restart sequence could come from an over-temperature
protection or an output over-voltage sensor.
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