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PDF RT7258 Data sheet ( Hoja de datos )
| Número de pieza | RT7258 | |
| Descripción | Synchronous Step-Down Converter | |
| Fabricantes | Richtek | |
| Logotipo |  |
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® RT7258
8A, 24V, 600kHz Step-Down Converter with Synchronous
Gate Driver
General Description
The RT7258 is a synchronous step-down DC/DC converter
with an integrated high side internal power MOSFET and
a gate driver for a low side external power MOSFET. It
can deliver up to 8A output current from a 4.5V to 24V
input supply. The RT7258's current mode architecture
allows the transient response to be optimized over a wider
input voltage and load range. Cycle-by-cycle current limit
provides protection against shorted outputs and soft-start
eliminates input current surge during start-up. The RT7258
is synchronizable to an external clock with frequency
ranging from 300kHz to 1.5MHz.
The RT7258 is available in WDFN-14L 4x3 and SOP-8
(Exposed Pad) packages.
Applications
z Point of Load Regulator in Distributed Power System
z Digital Set top Boxes
z Personal Digital Recorders
z Broadband Communications
z Flat Panel TVs and Monitors
Features
z 4.5V to 24V Input Voltage Range
z 8A Output Current
z 45mΩ Internal High Side N-MOSFET
z Current Mode Control
z 600kHz Switching Frequency
z Adjustable Output from 0.808V to 15V
z Up to 95% Efficiency
z Internal Compensation
z Stable with Ceramic Capacitors
z Synchronous External Clock : 300kHz to 1.5MHz
z Cycle-by-Cycle Current Limit
z Input Under Voltage Lockout
z Output Under Voltage Protection
z Power Good Indicator
z Thermal Shutdown Protection
z RoHS Compliant and Halogen Free
Simplified Application Circuit
VIN
Power Good
Chip Enable
RT7258
VIN BOOT
CIN
SW
VCC
CVCC
BG
PGOOD
FB
EN/SYNC
GND
CBOOT
L
Q1
VOUT
R1
COUT
R2
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7258-00 January 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
Free Datasheet http://www.datasheet4u.com/
1 page  
RT7258
Absolute Maximum Ratings (Note 1)
z Supply Input Voltage, VIN ------------------------------------------------------------------------------------------ –0.3V to 26V
z Switching Voltage, SW -------------------------------------------------------------------------------------------- –0.3V to (VIN + 0.3V)
z BOOT to SW --------------------------------------------------------------------------------------------------------- –0.3V to 6V
z All Other Voltage ---------------------------------------------------------------------------------------------------- −0.3V to 6V
z Power Dissipation, PD @ TA = 25°C
WDFN-14L 4x3 ------------------------------------------------------------------------------------------------------- 1.667W
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------- 1.333W
z Package Thermal Resistance (Note 2)
WDFN-14L 4x3, θJA ------------------------------------------------------------------------------------------------- 60°C/W
WDFN-14L 4x3, θJC ------------------------------------------------------------------------------------------------- 7.5°C/W
SOP-8 (Exposed Pad), θJA ---------------------------------------------------------------------------------------- 75°C/W
SOP-8 (Exposed Pad), θJC --------------------------------------------------------------------------------------- 15°C/W
z Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------- 260°C
z Junction Temperature ----------------------------------------------------------------------------------------------- 150°C
z Storage Temperature Range -------------------------------------------------------------------------------------- –65°C to 150°C
z ESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------------- 2kV
MM (Machine Mode) ------------------------------------------------------------------------------------------------ 200V
Recommended Operating Conditions (Note 4)
z Supply Input Voltage, VIN ------------------------------------------------------------------------------------------ 4.5V to 24V
z Junction Temperature Range -------------------------------------------------------------------------------------- −40°C to 125°C
z Ambient Temperature Range -------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Shutdown Supply Current
Supply Current
Reference Voltage
Feedback Current
High Side Switch On Resistance
High Side Switch Current Limit
VREF
IFB
RDS(ON)
VEN = 0V
VEN = 3V, VFB = 1V
4.5V ≤ VIN ≤ 24V
VFB = 0.8V
BOOT − SW = 4.8V
Oscillation Frequency
Short Circuit Oscillation Frequency
Maximum Duty Cycle
Minimum On-Time
fOSC1
fOSC2
DMAX
tON
VFB = 0V
VFB = 0.6V
VFB = 1V
Input Under Voltage Lockout Threshold VUVLO
Input Under Voltage Lockout Threshold
Hysteresis
ΔVUVLO
EN Threshold
Voltage
Logic-High
Logic-Low
VIH
VIL
Min Typ
-- 1
-- 0.9
0.796 0.808
-- 10
-- 45
-- 13
-- 600
-- 190
-- 90
-- 100
4 4.2
Max
--
--
0.82
--
--
--
--
--
--
--
4.4
Unit
μA
mA
V
nA
mΩ
A
kHz
kHz
%
ns
V
-- 400 -- mV
2 -- 5.5
V
-- -- 0.4
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7258-00 January 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
Free Datasheet http://www.datasheet4u.com/
5 Page 
RT7258
Application Information
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
VOUT
FB
RT7258
GND
R1
R2
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
VOUT
=
VREF
⎛⎜⎝1+
R1
R2
⎞⎟⎠
Where VREF is the feedback voltage (0.808V typ.).
External Bootstrap Diode
Connect a 1μF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65% .The bootstrap diode can be a
low cost one such as IN4148 or BAT54. The external 5V
can be a 5V fixed input from system or a 5V output of the
RT7258. Note that the external boot voltage must be lower
than 5.5V.
5V
BOOT
RT7258
SW
1µF
Figure 2. External Bootstrap Diode
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT7258 quiescent current drops to lower than
3μA. Driving the EN pin high (2V < EN < 5.5V) will turn on
the device again. For external timing control, the EN pin
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS7258-00 January 2012
can also be externally pulled high by adding a REN resistor
and CEN capacitor from the VIN pin (see Figure 3).
EN
REN
VIN
EN
RT7258
CEN
GND
Figure 3. Enable Timing Control
An external MOSFET can be added to implement digital
control on the EN pin, as shown in Figure 4. In this case,
a 100kΩ pull-up resistor, REN, is connected between VIN
pin and the EN pin. MOSFET Q2 will be under logic control
to pull down the EN pin.
REN
100k
VIN
EN
EN Q2 RT7258
GND
Figure 4. Digital Enable Control Circuit
The chip starts to operate when VIN rises to 4.2V (UVLO
threshold). During the VIN rising period, if an 8V output
voltage is set, VIN is lower than the VOUT target value and
it may cause the chip to shut down. To prevent this
situation, a resistive voltage divider can be placed between
the input voltage and ground and connected to the EN pin
to adjust enable threshold, as shown in Figure 5. For
example, the setting VOUT is 8V and VIN is from 0V to
12V, when VIN is higher than 10V, the chip is triggered to
enable the converter. Assume REN1 = 50kΩ. Then,
REN2
=
(REN1
(VIN_S
x
−
VEN_T )
VEN_T )
where VEN_T is the enable comparator's logic-high reference
threshold voltage (1.7V) and VIN_S is the target turn on
input voltage (10V in this example). According to the
equation, the suggested resistor R EN2 is 10.2kΩ.
REN1
VIN
EN
REN2
RT7258
GND
Figure 5. Resistor Divider for Lockout Threshold Setting
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
Free Datasheet http://www.datasheet4u.com/
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet RT7258.PDF ] | |
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