MP1492 – 2A, 4.2V-16V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER
HS-FET deviates from its intended location and
To realize the stability when no external ramp is
used, usually the ESR value should be chosen
as follow:
produces jitter. It is necessary to understand that
there is a relationship between a system’s
stability and the steepness of the VFB ripple’s
downward slope. The slope steepness of the VFB
ripple dominates in noise immunity. The
magnitude of the VFB ripple doesn’t directly affect
the noise immunity directly.
TSW
TON
2
+
0.7× π
(3)
RESR
≥
COUT
TSW is the switching period.
Ramp with small ESR Cap
When the output capacitors are ceramic ones,
the ESR ripple is not high enough to stabilize the
system, and external ramp compensation is
needed. Skip to application information section
for design steps with small ESR caps.
L
Vo
Figure 3—Jitter in PWM Mode
SW
C4
R4
R9
R1
R2
IR4
IC4
IFB
Ceramic
FB
Figure 6—Simplified Circuit in PWM Mode
with External Ramp Compensation
Figure 4—Jitter in Skip Mode
Ramp with Large ESR Cap
In PWM mode, an equivalent circuit with HS-FET
off and the use of an external ramp
compensation circuit (R4, C4) is simplified in
Figure 6. The external ramp is derived from the
inductor ripple current. If one chooses C4, R9,
R1 and R2 to meet the following condition:
In the case of POSCAP or other types of
capacitor with larger ESR is applied as output
capacitor. The ESR ripple dominates the output
ripple, and the slope on the FB is quite ESR
related. Figure 5 shows an equivalent circuit in
PWM mode with the HS-FET off and without an
external ramp circuit. Turn to application
information section for design steps with large
ESR caps.
⎛
⎜
⎝
⎞
⎟
⎠
1
1
5
R1 ×R2
R1 + R2
(4)
<
×
+ R9
2π×FSW ×C4
Where:
SW
L
Vo
IR4 = IC4 + IFB ≈ IC4
(5)
ESR
POSCAP
And the ramp on the VFB can then be estimated
as:
R1
R2
FB
V − VO
R1 //R2
IN
(6)
VRAMP
=
×TON ×
R4 ×C4
R1 //R2 +R9
The downward slope of the VFB ripple then
follows
Figure 5—Simplified Circuit in PWM Mode
without External Ramp Compensation
MP1492 Rev. 1.1
3/13/2012
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