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PDF RT8238A Data sheet ( Hoja de datos )
| Número de pieza | RT8238A | |
| Descripción | Single Synchronous Buck PWM Controller | |
| Fabricantes | RICHTEK | |
| Logotipo |  |
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RT8238A
Single Synchronous Buck PWM Controller
General Description
The RT8238A PWM controller provides high efficiency,
excellent transient response, and high DC output accuracy
needed for stepping down high voltage batteries to
generate low voltage CPU core, I/O, and chipset RAM
supplies in notebook computers.
The constant on-time PWM control scheme handles wide
input/output voltage ratios with ease and provides 100ns
“instant-on” response to load transients while maintaining
a relatively constant switching frequency.
The RT8238A achieves high efficiency at a reduced cost
by eliminating the current sense resistor found in
traditional current mode PWMs. Efficiency is further
enhanced by its ability to drive very large synchronous
rectifier MOSFETs and enter diode emulation mode at
light load condition. The Audio Skipping Mode (ASM)
setting maintains the switching frequency above 25kHz,
which eliminates noise in audio applications. The buck
conversion allows this device to directly step down high
voltage batteries at the highest possible efficiency. The
RT8238A is intended for CPU core, chipset, DRAM, or
other low voltage supplies as low as 0.5V. The RT8238A
is available in a WQFN-12L 2x2 package.
Ordering Information
RT8238A
Package Type
QW : WQFN-12L 2x2 (W-Type)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Z : ECO (Ecological Element with
Halogen Free and Pb free)
Note :
Richtek products are :
` RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
` Suitable for use in SnPb or Pb-free soldering processes.
Features
z Built in 1% 0.5V Reference Voltage
z Adjustable 0.5V to 3.3V Output Range
z Quick Load Step Response within 100ns
z 4700ppm/°C Programmable Current Limit by Low
Side RDS(ON) Sensing
z 4.5V to 26V Battery Input Range
z Resistor Programmable Frequency
z Internal Ramp Current Limit Soft-Start Control
z Drives Large Synchronous Rectifier FETs
z Integrated Boost Switch
z Over/Under Voltage Protection
z Thermal Shutdown
z Power Good Indicator
z RoHS Compliant and Halogen Free
Applications
z Notebook Computers
z CPU Core Supply
z Chipset/RAM Supply as Low as 0.5V
z Generic DC/DC Power Regulator
Pin Configurations
(TOP VIEW)
12 11 10
LGATE
PHASE
UGATE
1
2
3
GND
13
9 PGOOD
8 EN
7 MODE
4 56
WQFN-12L 2x2
RT8238A-06 October 2011
www.richtek.com
1
1 page  
RT8238A
Electrical Characteristics
(VCC = 5V, VIN = 15V, VEN = 5V, VMODE = 5V, RTON = 500kΩ, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min Typ
PWM Controller
VCC Quiescent Supply Current IQ
VCC Shutdown Current
TON Operating Current
TON Shutdown Current
CS Shutdown Current
FB Error Comparator
Threshold Voltage
FB Input Bias Current
Output Voltage Range
ISHDN
On-Time
Minimum Off-Time
FB forced above the regulation
point, VEN = 5V
VCC Current, VEN = 0V
RTON = 500kΩ
RTON = 500kΩ
CS pull to GND
VCC = 4.5 o 5.5V, DEM
VFB = 0.5V
VIN =15V, VPHASE = 1.25V,
VMODE = 0V
VMODE = 0V, FB = 0.45V
-- 0.5
-- --
-- 30
-- --
-- --
495 500
−1 0.1
0.5 --
267 334
250 400
Current Sensing Threshold
CS Source Current
CS Source Current TC
VCS = 0.5V to 2V
On the basis of 25°C
9 10
-- 4700
Zero Crossing Threshold
ASM Min Frequency
Protection Function
Current Limit Threshold
UVP Threshold
VMODE >1.8V, Phase − GND
VMODE = 2.5V
GND − PHASE, VCS = 1V
UVP Detect, FB Falling Edge
−10 --
-- 25
85 100
60 70
OVP Threshold
OVP Detect, FB Rising Edge
120 125
OV Fault Delay
VCC Power On Reset (POR)
Threshold
POR Threshold Hysteresis
Current Limit Ramp at Soft
Start
UV Blank Time
FB forced above OV threshold
Rising Edge
Enable to current limit threshold =
50mV
From EN signal going high
-- 5
3.7 3.9
-- 100
-- 900
-- 4.5
Thermal Shutdown
Thermal Shutdown Hysteresis
Driver On-Resistance
TSD
ΔTSD
-- 150
-- 10
UGATE Driver Source
UGATE Driver Sink
LGATE Driver Source
LGATE Driver Sink
RUGATEsr
BOOT − PHASE forced to 5V,
UGATE High State
RUGATEsk
BOOT − PHASE forced to 5V,
UGATE Low State
RLGATEsr LGATE High State
RLGATEsk LGATE Low State
-- 2.5
-- 1.5
-- 2.5
-- 0.8
Max Unit
1.25
1
--
1
1
505
1
3.3
401
550
mA
μA
μA
μA
μA
mV
μA
V
ns
ns
11 μA
-- ppm/°C
5 mV
-- kHz
115 mV
80 %
130 %
-- μs
4.2 V
-- mV
-- μs
-- ms
-- °C
-- °C
5Ω
3Ω
5Ω
1.5 Ω
RT8238A-06 October 2011
To be continued
www.richtek.com
5
5 Page 
IL
Slope = (VIN -VOUT) / L
IPEAK
ILOAD = IPEAK / 2
0 tON
t
Figure 1. Boundary Condition of CCM/DEM
The switching waveforms may appear noisy and
asynchronous when light loading causes diode emulation
operation, but this is a normal operating condition that
results in high light load efficiency. Trade offs in DEM
noise vs. light load efficiency is made by varying the
inductor value. Generally, low inductor values produce a
broader efficiency vs. load curve, while higher values result
in higher full load efficiency (assuming that the coil
resistance remains fixed) and less output voltage ripple.
The disadvantages for using higher inductor values include
larger physical size and degrade load transient response
(especially at low input voltage levels).
Audio-Skipping Mode
When the MODE pin is pulled to 2.5V, the controller
operates in audio skipping mode with a minimum switching
frequency of 25kHz. This mode eliminates audio-frequency
modulation that would otherwise be present when a lightly
loaded controller automatically skips pulses. In audio
skipping mode, the low side switch gate driver signal is
ORed with an internal oscillator (>25kHz). Once the
internal oscillator is triggered, the audio skipping controller
pulls LGATE logic high, turning on the low side MOSFET
to induce a negative inductor current. After the output
voltage rises above VREF, the controller turns off the low
side MOSFET (LGATE pulled logic low) and triggers a
constant on-time operation (UGATE driven logic high).
When the on-time operation expires, the controller re-
enables the low side MOSFET until the inductor current
drops below the zero-crossing threshold.
Forced-CCM Mode
The low noise, forced-CCM mode (MODE = GND) disables
the zero-crossing comparator, which controls the low side
switch on-time. This causes the low side gate drive
RT8238A-06 October 2011
RT8238A
waveform to become the complement of the high side
gate drive waveform. This in turn causes the inductor
current to reverse at light loads as the PWM loop to
maintain a duty ratio VOUT/VIN. The benefit of forced-CCM
mode is to keep the switching frequency fairly constant,
but it comes at a cost. The no load battery current can be
up to 10mA to 40mA, depending on the external
MOSFETs.
Current Limit Setting (OCP)
The RT8238A has cycle-by-cycle current limiting control.
The current limit circuit employs a unique “valley” current
sensing algorithm. If PHASE voltage plus the current-limit
threshold is below zero, the PWM is not allowed to initiate
a new cycle (Figure 2). In order to provide both good
accuracy and a cost effective solution, the RT8238A
supports temperature compensated MOSFET RDS(ON)
sensing. The CS pin should be connected to GND through
the trip voltage setting resistor, RCS. With the 10μA CS
terminal source current, ICS, and the setting resistor, RCS
the CS trip voltage, VCS, can be calculated as shown in
the following equation.
VCS (mV) = RCS (kΩ) x 10 (μA) x (1 / 10)
Inductor current is monitored by the voltage between the
PGND pin and the PHASE pin, so the PHASE pin should
be connected to the drain terminal of the low side
MOSFET. ICS has positive temperature coefficient to
compensate the temperature dependency of the RDS(ON).
PGND is used as the positive current sensing node so
PGND should be connected to the source terminal of the
bottom MOSFET.
As the comparison is done during the OFF state, VCS
sets the valley level of the inductor current. Thus, the
load current at over current threshold, ILOAD_OC, can be
calculated as follows.
ILOAD_OC
=
VCS
RDS(ON)
+
IRipple
2
=
VCS
RDS(ON)
+
2
1
×L
×
f
×
( VIN
−
VOUT ) × VOUT
VIN
www.richtek.com
11
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| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet RT8238A.PDF ] | |
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