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PDF RT8541A Data sheet ( Hoja de datos )
| Número de pieza | RT8541A | |
| Descripción | Boost DC/DC Converter | |
| Fabricantes | Richtek | |
| Logotipo |  |
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®
RT8541/A
90V Boost DC/DC Converter with APD Current Monitor
General Description
The RT8541/A is a constant frequency current mode PWM
step-up DC/DC converter designed to bias avalanche
photodiodes (APD) at high voltage in optical receivers. It
includes a voltage doubler charge pump to significantly
reduce the noise level at output up to 90V. The RT8541A
switching frequency can be adjusted by a resistor while
RT8541 is internally set at 650kHz. The RT8541/A has a
built-in APD current mirror that delivers 1/5 of the APD
current with high accuracy for monitoring the APD current
from 0.25μA to 2.5mA. This current can be used as a
reference to provide a digital programmed output voltage
via the CTRL pin. To protect the optical receiver system,
the RT8541/A provides a built-in adjustable APD current
limit function by sensing a 200mV voltage threshold
between VOUT2 and MONIN pins. Only one resistor is
needed to program the desired current limit. A charge pump
voltage doubler is designed to achieve significantly lower
noise level at the APD comparing to a simple boost
converter type bias circuit.
The RT8541/A also features a built-in 9ms soft-start
function to eliminate the inrush current during start-up
interval. The RT8541/A provides a low cost optical receiver
solution and can fit in very small PCB area.
The RT8541/A is available in the WQFN-16L 3x3 package.
Features
Integrated Schottky Diodes for Charge Pump
Voltage Doubler
50V, 400mA Internal Switch
High Side Low Noise and Accurate APD Current
Monitor
650kHz Fixed Switching Frequency for RT8541 and
Adjustable Switching Frequency for RT8541A
Wide VIN Range : 2.7V to 16V
Adjustable APD Current Limit with External Resistor
Low Shutdown Current <1μA
Built-In Soft-Start
CTRL Pin Allows Output Adjustment with no Polarity
Inversion
Thin 16-Lead WQFN Package
RoHS Compliant and Halogen Free
Applications
APD Bias
PIN Diode Bias
Optical Receivers and Modules
Fiber Optic Network Equipment
Simplified Application Circuit
L C1
VIN
CIN
SW PUMP
VIN RT8541/A
R5
CTRL
MONIN
SHDN
VOUT2
VC
R4
FSET
VOUT1
RSET
C4
GND
FB
MON
APD
R6
C2
C3
CFB R2
VOUT
COUT
R1
R3
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8541/A-01 June 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
1 page  
RT8541/A
Absolute Maximum Ratings (Note 1)
VIN, SHDN, FSET, CTRL, MON ---------------------------------------------------------------------------------------- 16V
VOUT1, SW ----------------------------------------------------------------------------------------------------------------- 50V
VOUT2, PUMP, MONIN, APD ------------------------------------------------------------------------------------------- 90V
FB, VC, FSET --------------------------------------------------------------------------------------------------------------- 5V
Power Dissipation, PD @ TA = 25°C
WQFN-16L 3x3 ------------------------------------------------------------------------------------------------------------- 3.33W
Package Thermal Resistance (Note 2)
WQFN-16L 3x3, θJA -------------------------------------------------------------------------------------------------------- 30°C/W
WQFN-16L 3x3, θJC ------------------------------------------------------------------------------------------------------- 7.5°C/W
Junction Temperature ------------------------------------------------------------------------------------------------------ 150°C
Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------- 260°C
Storage Temperature Range --------------------------------------------------------------------------------------------- −65°C to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) ----------------------------------------------------------------------------------------------- 2kV
MM (Machine Model) ------------------------------------------------------------------------------------------------------ 200V
Recommended Operating Conditions (Note 4)
Supply Input Voltage, VIN ------------------------------------------------------------------------------------------------ 3V to 12V
Junction Temperature Range --------------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range --------------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 5V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Operating Voltage
Feedback Voltage
VFB CTRL = 1.5V
Feedback Line Regulation
VIN = 3V to 10V
FB Pin Bias Current
Supply Current
Shutdown Current
IVIN FB = 1.3V, VSHDN = 2V
VSHDN = 0V, Not Switching
Switching Frequency
RSET = NC (RT8541)
fSW
RSET = 30k (RT8541A)
Maximum Duty Cycle
Switch Current Limit
Switch VCESAT
DMAX
ILIM
fSW = 650kHz
ISW = 150mA
Switch Leakage Current
SW = 5V
Schottky Forward Voltage
ISCHOTTKY = 150mA
Schottky Reverse Leakage
VOUT1 SW = 50V
Min Typ Max
2.7 -- 16
1.215 1.235 1.255
-- 0.025 0.07
-- 30 100
-- 3.3 4
-- 0.1 0.5
-- 650 --
-- 650 --
95 --
--
320 400 480
-- 130 220
-- -- 2
-- 720 --
-- -- 5
Unit
V
V
%/V
nA
mA
A
kHz
%
mA
mV
A
mV
A
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8541/A-01 June 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
5 Page 
RT8541/A
Output Capacitor Selection
The output ripple voltage is an important index for
estimating chip performance. This portion consists of two
parts. One is the product of the inductor current with the
ESR of the output capacitor, while the other part is formed
by the charging and discharging process of the output
capacitor. As shown in Figure 1, ΔVOUT2 can be evaluated
based on the ideal energy equalization. According to the
definition of Q, the Q value can be calculated as the
following equation :
Q=
1
2
IIN
1
2
IL
IAPD
IIN
1
2
IL
IAPD
VIN 1
VOUT2 fSET
= COUT VOUT2
where fSET is the switching frequency, and ΔIL is the
inductor ripple current. Bring COUT to the left side to
estimate the value of ΔVOUT2 according to the following
equation :
VOUT2
=
D IAPD
COUT fSET
where D is the duty cycle and η is the Boost converter
efficiency. Finally, taking ESR into account, the overall
output ripple voltage can be determined by the following
equation :
VOUT
=
IIN
ESR
D IOUT
COUT fOSC
For applications with out voltage less than 45V, intermediate
output pin VOUT1 can directly serve as the output pin.
Typically use a 2μF capacitor for output voltage less than
25V and 1μF capacitor for output voltage between 25V
and 45V. When output voltage goes beyond 45V, a charge
pump must be formed with cascaded 0.47μF capacitors
C1 and C2 at the output nodes. A typical 0.1μF capacitor
is used as the flying capacitor CFLY to form the charge
pump. Always use a capacitor with sufficient voltage rating.
Either ceramic or solid tantalum capacitors may be used
for the input decoupling capacitor, which should be placed
as close as possible to the RT8541. A 1μF capacitor is
sufficient for most applications.
Input Capacitor Selection
Low ESR ceramic capacitors are recommended for input
capacitor applications. Low ESR will effectively reduce
the input voltage ripple caused by switching operation. A
10μF capacitor is sufficient for most applications.
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8541/A-01 June 2015
Nevertheless, this value can be decreased for lower output
current requirement. Another consideration is the voltage
rating of the input capacitor which must be greater than
the maximum input voltage.
APD Current Monitor
The power supply switching noise associated with a
switching power supply can interfere with the photodiode
DC measurement. To suppress this noise, a 0.1μF
capacitor is recommended at APD pin. An additional output
low-pass filter, a 10k resistor and a 10nF capacitor in
parallel at MON pin might limit the measurement accuracy
of low level signals. For applications requiring fast current
monitor response time, a RC low-pass filter at MONIN pin
is used to replace the 0.1μF capacitor at APD pin to reduce
the power supply noise and other wide band noise.
APD Short Current Protection
In some applications, a long cable or wire is used to
connect the RT8541/A to APD. When APD is shorted to
GND, APDpin voltage might ring below ground and damage
the internal circuitry. To prevent damage from short-circuit
event, a 100Ω resistor must be added between MONIN
and VOUT2 pins. The APD short current is 200mV/R6.
For a typical 2mA short current protection of APD, the R6
should be set at 100Ω.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TAis
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
WQFN-16L 3x3 package, the thermal resistance, θJA, is
30°C/W on a standard JEDEC 51-7 four-layer thermal test
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
11 Page | ||
| Páginas | Total 13 Páginas | |
| PDF Descargar | [ Datasheet RT8541A.PDF ] | |
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