rt9259 Richtek Technology Corporation, rt9259 Datasheet - Page 10

• Manufacturer Part Number: rt9259
• Description: Synchronous Buck Dc-dc Linear Power Controller
• Manufacturer: Richtek Technology Corporation

RT9259
If MOSFET with R
threshold current is about 25A. Once OCP is triggered,
the RT9259 enters hiccup mode and re-soft starts again.
The RT9259 shuts down after 4 time OCP hiccups.
Feedback Compensation
The RT9259 is a voltage mode controller. The control loop
is a single voltage feedback path including a compensator
and modulator as shown Figure 8. The modulator consists
of the PWM comparator and power stage. The PWM
comparator compares error amplifier EA output (COMP)
with oscillator (OSC) sawtooth wave to provide a pulse-
width modulated (PWM) with an amplitude of V
PHASE node. The PWM wave is smoothed by the output
filter L
and fed to the inverting input of the error amplifier.
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10
Figure 7. Shorted then Start Up (Extended Figure 3)
OUT
Inductor Current
and C
(20A/Div)
Figure 6. Shorted then Start Up
OUT
. The output voltage (V
DS(ON)
Time (2.5ms/Div)
Time (5μs/Div)
= 16mΩ is used, the OCP
(20A/Div)
OUT
(5V/Div)
(5V/Div)
UGATE
LGATE
I
) is sensed
L
IN
at the
A well-designed compensator regulates the output voltage
to the reference voltage V
and good stability.
In order to achieve fast transient response and accurate
output regulation, an adequate compensator design is
necessary. The goal of the compensation network is to
provide adequate phase margin (greater than 45 degrees)
and the highest 0dB crossing frequency. It is also
recommended to manipulate loop frequency response that
its gain crosses over 0dB at a slope of −20dB/dec.
1) Modulator Frequency Equations
The modulator transfer function is the small-signal transfer
function of V
amplifier output. This transfer function is dominated by a
DC gain, a double pole, and a zero as shown in Figure 10.
The DC gain of the modulator is the input voltage (V
divided by the peak to peak oscillator voltage V
output LC filter introduces a double pole, 40dB/decade
gain slope above its corner resonant frequency, and a total
phase lag of 180 degrees. The resonant frequency of the
LC filter expressed as :
f
LC
ΔV
OSC
=
2
π
OSC
L
Comparator
OUT
OUT
PWM
COMP
COMP
1
C1
×
/V
-
+
Figure 8. Closed Loop
EA
Z
C
FB
COMP
EA
C2
OUT
-
+
REF
-
+
R2
(output voltage over the error
REF
REF
FB
Z
Driver
Driver
FB
DS9259-03T00 August 2007
with fast transient response
C3
Z
R1
IN
Z
R3
IN
V
IN
PHASE
V
OUT
C
L
OUT
ESR
OSC
V
. The
OUT
IN
)