rt9238 ETC-unknow, rt9238 Datasheet - Page 16

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

RT9238
elements. The voltage spikes can degrade efficiency,
radiate noise into the circuit, and lead to device over-
voltage stress. Careful component layout and printed
circuit design minimizes the voltage spikes in the
converter. Consider, as an example, the turn-off
transition of the upper MOSFET. Prior to turn-off, the
upper MOSFET was carrying the full load current.
During the turn-off, current stops flowing in the upper
MOSFET and is picked up by the lower MOSFET or
Schottky diode. Any inductance in the switched
current path generates a large voltage spike during
the switching interval. Careful component selection,
tight layout of the critical components, and short,
wide circuit traces minimize the magnitude of voltage
spikes.
There are two sets of critical components in a DC-DC
converter using an RT9238 controller. The switching
power components are the most critical because they
switch large amounts of energy, and as such, they
tend to generate equally large amounts of noise. The
critical small signal components are those connected
to sensitive nodes or those supplying critical bypass
current.
The power components and the controller IC should
be placed first. Locate the input capacitors, especially
the high-frequency ceramic de-coupling capacitors,
close to the power switches. Locate the output
inductor
MOSFETs and the load. Locate the PWM controller
close to the MOSFETs.
The critical small signal components include the
bypass capacitor for VCC and the soft-start capacitor,
C
connecting pins on the control IC. Minimize any
leakage current paths from any SS node, since the
internal current source is only 28µA.
A multi-layer printed circuit board is recommended.
Fig.7
components in the converter. Note that the capacitors
C
physical capacitors. Dedicate one solid layer for a
ground plane and make all critical
www.richtek-ic.com.tw
16
SS
IN
. Locate these components close to their
and C
shows
and
OUT
the
output
each could represent numerous
connections
capacitors
of
between
the
critical
Preliminary
the
+5V
IN
component ground connections with vias to this layer.
Dedicate another solid layer as a power plane and
break this plane into smaller islands of common
voltage levels. The power plane should support the
input power and output power nodes. Use copper
filled polygons on the top and bottom circuit layers for
the PHASE node, but do not unnecessarily oversize
this particular island. Since the PHASE node is
subject to very high dV/dt voltages, the stray
capacitor formed between these island and the
surrounding circuitry will tend to couple switching
noise. Use the remaining printed circuit layers for
small signal wiring. The wiring traces from the control
IC to the MOSFET gate and source should be sized
to carry 2A peak currents.
PWM1 Controller Feedback Compensation
The PWM controller uses voltage-mode control for
output
compensation scheme is used with an active
capacitor and a passive resistor shown in Fig.8. The
zero FZ1 is fixed at about 1kHz to compensate the
output ‘s LC pole FLC. The compensation is to
+3.3V
V
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT PLANE LAYER
VIA/THROUGH-HOLE CONNECTION TO GROUND PLANE
OUT3
C
V
SS24,13
OUT2
L
IN
C
IN
C
IN
OUT3
regulation.
Q3
C
OUT2
Q4
+12V
Fig.7 Layout Planning
DRIVE2
SS24
SS13
DRIVE3
VCC
C
RT9238
VCC
PGND
GND
UGATE
DRIVE4
OCSET
PHASE
LGATE
An
DS9238-01 July 2001
internal
C
Q2
Q5
OCSET
C
OUT4
Q1
R
V
L
OCSET
OUT4
OUT
CR1
C
pole-zero
OUT1
V
OUT1