rt9238 ETC-unknow, rt9238 Datasheet - Page 17

• Manufacturer Part Number: rt9238
• Description: Vrm 8.5 Pwm And Triple Linear Power System Controller
• Manufacturer: ETC-unknow

provide closed loop transfer function with 0 dB
crossing frequency and adequate phase margin. For
some suggestion LC combinations are L/C =
4µH/6000µF, 3uH/8000uF or 2µH/12000µF.
shows an asymptotic plot of the DC-DC converter’s
gain vs. frequency.
Transient Response
Modern micro-processsor’s power supply request a
voltage droop at load transient response and a
loadlines at static state load changing as shown in
Fig.10, and Fig.11.
DS9238-01 July 2001
100
-20
-40
-60
80
60
40
20
0
10
Fig.8 Error Amplifier Compensation
20log
V
CORE
Fig.10 Transient Response
MODULATOR
V
V
∆ I
IN
P-P
100
Fig.9 PWM Bode Plot
Ripple
O
GAIN
_
EA
F
F
LC
Z1
1K
F
ESR
10K
Active Cap
COMPENSATION
F
100K
P1
CLOSED
LOOP
1M
GAIN
Fig.9
Preliminary
REF
Two things have to be completed at load transient
response, first is to sense the instantaneous load
current, second is real time to drop the V
based on the loadlines
conductance (Gm) amplifier sample the on-state drop
across the lower MOSFET per clock cycle, the
voltage drop is simply R
step-down DC-DC converter, the IL is relative to load
current. The transferred rate of the Gm amplifier is
1/3k, this mean the current output of Gm amplifier is
Fig.12 shown the voltage droop tuning circuit, a
DACOUT buffer amplifier connect a RF resistor to the
non-inverting input of the PWM error amplifier, when
load transient happened, the feedback I
fitted to the non-inverting input of the PWM error
amplifier to drop the VCORE voltage. The drop
voltage is equal to
Sample Hold
COMP
Low Side Current Sense
Error Amp
VNI
I
V
Volt
1.70
1.60
SENSE
Isense
DROOP
Control
Logic
Gm
_
DAC Buffer
_
_
V
= R
Fig.12 Voltage Droop Tuning
CORE
= I
Fig.11 V
RS
RS
DS(ON)
SENSE
10
REFOUT
Minimum
RF
Reference Output
PHASE
LG
x IL/3k.
UG
CORE
x RF
DS-ON X
.
V
CORE
20
A RT9238 internal trans-
Rds-on
Load Line
IM2
IM1
Maximum
inductor current IL. In
V
V5V
www.richtek-ic.com.tw
CORE
30
RT9238
Typical
IL
SENSE
CORE
A
+
voltage
current
VCORE
17