Datasheet

RAMP
VCC
C
RAMP
R
RAMP
LM5576, LM5576-Q1
SNVS447I –JANUARY 2007–REVISED APRIL 2013 www.ti.com
Proper selection of the RAMP capacitor depends upon the selected value of the output inductor. The value of
C
RAMP
can be selected from: C
RAMP
= L x 10
-5
, where L is the value of the output inductor in Henrys. With this
value, the scale factor of the emulated current ramp will be approximately equal to the scale factor of the DC
level sample and hold ( 0.5 V / A). The C
RAMP
capacitor should be located very close to the device and
connected directly to the pins of the IC (RAMP and AGND).
For duty cycles greater than 50%, peak current mode control circuits are subject to sub-harmonic oscillation.
Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow pulses at the switch
node. Adding a fixed slope voltage ramp (slope compensation) to the current sense signal prevents this
oscillation. The 25µA of offset current provided from the emulated current source adds some fixed slope to the
ramp signal. In some high output voltage, high duty cycle applications, additional slope may be required. In these
applications, a pull-up resistor may be added between the V
CC
and RAMP pins to increase the ramp slope
compensation.
For V
OUT
> 7.5V:
Calculate optimal slope current, I
OS
= V
OUT
x 5µA/V.
For example, at V
OUT
= 10V, I
OS
= 50µA.
Install a resistor from the RAMP pin to V
CC
:
R
RAMP
= V
CC
/ (I
OS
- 25µA)
Figure 15. R
RAMP
to V
CC
for V
OUT
> 7.5V
Note that the emulated ramp signal on C
RAMP
is applied to the current limit comparator as described in the
Current Limit section below. Increasing the ramp slope will result in lower current limit threshold. This can lower
the output current capability of the part to less than 3A in some conditions. The resulting current limit threshold
can be calculated by the following equation:
where
• V
CL
= 2.1V
• gm = 5µA/V
• Ioffset = 25µA
• A x Rs = 0.5V/A
• Vcc = 7V
• T = switching period
• D = duty cycle (approximately V
OUT
/ V
IN
)
• L = inductor value
• C
RAMP
= ramp capacitor value
• R
RAMP
= ramp resistor value (3)
If the recommended C
RAMP
and R
RAMP
values are used, then the following simplified equation calculates the
current limit threshold:
(4)
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