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RT6576A/B
Dual-Channel Synchronous DC/DC Step-Down Controller
with 5V/3.3V LDOs
General Description
Features
The RT6576A/B is a dual-channel step-down controller
generating supply voltages for battery-powered systems.
It includes two Pulse-Width Modulation (PWM) controllers
adjustable from 2V to 5.5V, and two fixed 5V/3.3V linear
regulators. Each linear regulator provides up to 100mA
output current and 3.3V linear regulator provides 1%
accuracy under 35mA. The RT6576A/B has an oscillator
output to drive the external charge pump application. Other
features include on-board power-up sequencing, a powergood output, internal soft-start, and soft-discharge output
that prevents negative voltage during shutdown.
A constant current ripple PWM control scheme operates
without sense resistors and provides 100ns response to
load transient. For maximizing power efficiency, the
RT6576A/B automatically switches to the diode-emulation
mode in light load applications. The RT6576A/B is available
in the WQFN-20L 3x3 package.
Support Connected Standby Mode for Ultrabook
CCRCOT Control with 100ns Load Step Response
PWM Maximum Duty Ratio > 98%
5V to 25V Input Voltage Range
Dual Adjustable Output :
CH1 : 2V to 5.5V
CH2 : 2V to 4V
5V/3.3V LDOs with 100mA Output Current
1% Accuracy on 3.3V LDO Output
Oscillator Driving Output for Charge Pump
Application
Internal Frequency Setting
300kHz/355kHz (CH1/CH2)
Internal Soft-Start and Soft-Discharge
4700ppm/°°C RDS(ON) Current Sensing
Independent Switcher Enable Control
Built-in OVP/UVP/OCP/OTP
Non-Latch UVLO
Power Good Indicator
RoHS Compliant and Halogen Free
Simplified Application Circuit
VIN
UGATE2
RT6576A/B
BOOT2
UGATE1
PHASE2
BOOT1
LGATE2
PHASE1
VIN
VOUT1
LGATE1
VOUT2
FB2
CS1
CS2
BYP1
LDO5
5V
FB1
Channel 1 Enable
EN1
Channel 2 Enable
EN2
VCLK
Copyright © 2022 Richtek Technology Corporation. All rights reserved.
DS6576A/B-03 April 2022
PGOOD
LDO3
PGOOD Indicator
3.3V
GND
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT6576A/B
Marking Information
Applications
Notebook and Sub-Notebook Computers
System Power Supplies
2-Cell to 4-Cell Li+ Battery-Powered Devices
RT6576AGQW
6V= : Product Code
YMDNN : Date Code
6V=YM
DNN
Ordering Information
RT6576A/B
RT6576BGQW
Pin 1 Orientation***
(2) : Quadrant 2, Follow EIA-481-D
Package Type
QW : WQFN-20L 3x3 (W-Type)
Lead Plating System
G : Green (Halogen Free and Pb Free)
6U= : Product Code
6U=YM
DNN
YMDNN : Date Code
Pin Configuration
(TOP VIEW)
EN1
VCLK
PHASE1
BOOT1
UGATE1
Pin Function With
A : LDO3 Always On
B : LDO3/LDO5 Always On
Note :
Richtek products are :
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
20 19 18 17 16
CS1
FB1
LDO3
FB2
CS2
1
15
2
14
GND
3
4
13
12
21
5
11
6
7
8
LGATE1
BYP1
LDO5
VIN
LGATE2
9 10
EN2
PGOOD
PHASE2
BOOT2
UGATE2
***Empty means Pin1 orientation is Quadrant 1
WQFN-20L 3x3
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is a registered trademark of Richtek Technology Corporation.
DS6576A/B-03 April 2022
RT6576A/B
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
CS1
Current Limit Setting. Connect a resistor to GND to set the threshold for Channel 1
synchronous RDS(ON) sense. The GND PHASE1 current limit threshold is 1/8th
the voltage seen at CS1 over a 0.2V to 2V range. There is an internal 10A
current source from LDO5 to CS1.
2
FB1
Feedback Voltage Input for Channel 1. Connect FB1 to a resistive voltage divider
from VOUT1 to GND to adjust output from 2V to 5.5V.
3
LDO3
3.3V Linear Regulator Output. It is always on when VIN is higher than VINPOR
threshold.
4
FB2
Feedback Voltage Input for Channel 2. Connect FB2 to a resistive voltage divider
from VOUT2 to GND to adjust output from 2V to 4V.
5
CS2
Current Limit Setting. Connect a resistor to GND to set the threshold for Channel 2
synchronous RDS(ON) sense. The GND PHASE2 current limit threshold is 1/8th
the voltage seen at CS2 over a 0.2V to 2V range. There is an internal 10A
current source from LDO5 to CS2.
6
EN2
Enable Control Input for Channel 2.
7
PGOOD
Power Good Indicator Output for Channel 1 and Channel 2. (Logical AND)
8
PHASE2
Switch Node of Channel 2 MOSFETs. PHASE2 is the internal lower supply rail for
the UGATE2 high-side gate driver. PHASE2 is also the current-sense input for the
Channel 2.
9
BOOT2
Bootstrap Supply for Channel 2 High-Side Gate Driver. Connect to an external
capacitor according to the typical application circuits.
10
UGATE2
High-Side Gate Driver Output for Channel 2. UGATE2 swings between PHASE2
and BOOT2.
11
LGATE2
Low-Side Gate Driver Output for Channel 2. LGATE2 swings between GND and
LDO5.
12
VIN
Power Input for 5V and 3.3V LDO Regulators and Buck Controllers.
13
LDO5
5V Linear Regulator Output. LDO5 is also the supply voltage for the low-side
MOSFET and analog supply voltage for the device.
14
BYP1
Switch-over Source Voltage Input for LDO5.
15
LGATE1
Low-Side Gate Driver Output for Channel 1. LGATE1 swings between GND and
LDO5.
16
UGATE1
High-Side Gate Driver Output for Channel 1. UGATE1 swings between PHASE1
and BOOT1.
17
BOOT1
Bootstrap Supply for Channel 1 High-Side Gate Driver. Connect to an external
capacitor according to the typical application circuits.
18
PHASE1
Switch Node of Channel 1 MOSFETs. PHASE1 is the internal lower supply rail for
the UGATE1 high-side gate driver. PHASE1 is also the current sense input for the
Channel 1.
19
VCLK
Oscillator Output for Charge Pump.
20
EN1
Enable Control Input for Channel 1.
21
GND
(Exposed Pad)
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
Copyright © 2022 Richtek Technology Corporation. All rights reserved.
DS6576A/B-03 April 2022
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT6576A/B
Function Block Diagram
BOOT1
BOOT2
UGATE1
UGATE2
PHASE2
PHASE1
LDO5
Channel 1
Buck
Controller
LGATE1
LDO5
Channel 2
Buck
Controller
LGATE2
FB1
CS1
FB2
CS2
BYP1
PGOOD
VCLK
OSC
GND
SW5 Threshold
BYP1
LDO5
Power-On
Sequence
Clear Fault Latch
EN1
REF
LDO3
LDO5
LDO3
EN2
BYP1
VIN
Operation
The RT6576A/B includes two constant on-time
synchronous step-down controllers and two linear
regulators.
Buck Controller
In normal operation, the high-side N-MOSFET is turned
on when the output is lower than VREF, and is turned off
after the internal one-shot timer expires. While the highside N-MOSFET is turned off, the low-side N-MOSFET is
turned on to conduct the inductor current until next cycle
begins.
Soft-Start
For internal soft-start function, an internal current source
charges an internal capacitor to build the soft-start ramp
voltage. The output voltage will track the internal ramp
voltage during soft-start interval.
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PGOOD
The power good output is an open-drain architecture. When
the two channels soft-start are both finished, the PGOOD
open-drain output will be high impedance.
Current Limit
The current limit circuit employs an unique “valley” current
sensing algorithm. If the magnitude of the current sense
signal at PHASE is above the current limit threshold, the
PWM is not allowed to initiate a new cycle. Thus, the
current to the load exceeds the average output inductor
current, the output voltage falls and eventually crosses
the under-voltage protection threshold, inducing IC
shutdown.
is a registered trademark of Richtek Technology Corporation.
DS6576A/B-03 April 2022
RT6576A/B
Over-Voltage Protection (OVP) & Under-Voltage
Protection (UVP)
The two channel output voltages are continuously
monitored for over-voltage and under-voltage conditions.
When the output voltage exceeds over-voltage threshold
(113% of VOUT), UGATE goes low and LGATE is forced
high. When it is less than 52% of reference voltage, undervoltage protection is triggered and then both UGATE and
LGATE gate drivers are forced low. The controller is latched
until ENx is reset or LDO5 is re-supplied.
Copyright © 2022 Richtek Technology Corporation. All rights reserved.
DS6576A/B-03 April 2022
LDO5 and LDO3
When the VIN voltage exceeds the POR rising threshold,
LDO3 will default turn-on. The LDO5 can be power on by
ENx. The linear regulator LDO5 and LDO3 provide 5V and
3.3V regulated output.
Switching Over
The BYP1 is connected to the Channel 1 output. After the
Channel 1 output voltage exceeds the set threshold
(4.66V), the output will be bypassed to the LDO5 output
to maximize the efficiency.
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RT6576A/B
Absolute Maximum Ratings
(Note 1)
VIN to GND ----------------------------------------------------------------------------------------------------------------BOOTx to GND
DC --------------------------------------------------------------------------------------------------------------------------- 5V, ILDO3 < 20mA
3.217
3.3
3.383
kHz
Soft-Start
Soft-Start Time
Current Sense
Internal Regulator
LDO5 Output Voltage
LDO3 Output Voltage
VLDO5
VLDO3
Copyright © 2022 Richtek Technology Corporation. All rights reserved.
DS6576A/B-03 April 2022
V
V
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RT6576A/B
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
LDO5 Output Current
ILDO5
VLDO5 = 4.5V, VBYP1 = GND,
VIN = 7.4V
100
175
--
mA
LDO3 Output Current
ILDO3
VLDO3 = 3V, VIN = 7.4V
100
175
--
mA
LDO5 Switch-over
Threshold to BYP1
VSWTH
Rising Edge at BYP1 Regulation
Point
--
4.66
--
V
LDO5 Switch-over
Equivalent Resistance
RSW
LDO5 to BYP1, 10mA
--
1.5
3
VCLK On-Resistance
RVCLK
Pull-up and Pull-down Resistance
--
10
--
VCLK Switching Frequency
f VCLK
--
260
--
kHz
Rising Edge
--
4.3
4.6
Falling Edge
3.7
3.9
4.1
Channel x Off
--
2.5
--
PGOOD Detect, VFBx Rising Edge
84
88
92
Hysteresis
--
8
--
PGOOD Leakage Current
High state, VPGOOD = 5.5V
--
--
1
A
PGOOD Output Low
Voltage
ISINK = 4mA
--
--
0.3
V
109
113
117
%
--
1
--
s
VCLK Output
UVLO
LDO5 UVLO Threshold
VUVLO5
LDO3 UVLO Threshold
VUVLO3
V
V
Power Good Indicator
PGOOD Threshold
VPGxTH
%
Fault Detection
OVP Trip Threshold
VOVP
FBx with Respect to Internal
Reference
OVP Propagation Delay
UVP Trip Threshold
VUVP
UVP Detect, FBx Falling Edge
47
52
57
%
UVP Shutdown Blanking
Time
tSHDN_UVP
From ENx Enable
--
4.5
--
ms
Thermal Shutdown
Thermal Shutdown
TSD
--
150
--
°C
Thermal Shutdown
Hysteresis
TSD
--
10
--
°C
Logic Inputs
ENx Threshold Logic-High
Voltage
Logic-Low
VENx_H
SMPS On
1.6
--
--
VENx_L
SMPS Off
--
--
0.4
RBST
LDO5 to BOOTx
--
80
--
V
Internal Boost Switch
Internal Boost Switch
On-Resistance
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is a registered trademark of Richtek Technology Corporation.
DS6576A/B-03 April 2022
RT6576A/B
Parameter
Symbol
Test Conditions
Min
Typ
Max
High State, VBOOTx VUGATEx = 0.25V,
VBOOTx VPHASEx = 5V
--
3
--
Low State, VUGATEx VPAHSEx =
0.25V, VBOOTx VPHASEx = 5V
--
2
--
High State, VLDO5 VLGATEx = 0.25V,
VLDO5 = 5V
--
3
--
Low State, VLGATEx GND = 0.25V
--
1
--
LGATEx Rising
--
20
--
UGATEx Rising
--
30
--
Unit
Power MOSFET Drivers
UGATEx On-Resistance
LGATEx On-Resistance
Dead-Time
RUGATEx
RLGATEx
tD
ns
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precautions are recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2022 Richtek Technology Corporation. All rights reserved.
DS6576A/B-03 April 2022
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
9
RT6576A/B
Typical Application Circuit
VIN
5V to 25V
R8
0
C1
10µF
C10
0.1µF
R4 0 16
Q1
BSC0909
NS
VOUT1
5V
R3 0
L1
3.3µH
C3
220µF
R5*
RT6576A
12
17
C2
0.1µF
UGATE2
VIN
BOOT2
Q3
BSC0909
NS
15
PHASE2
BOOT1
R9 0
C11
0.1µF
8
L2
2.2µH
Q4
BSC0909
NS
11
C12
10µF
C13
10µF
VOUT2
3.3V
C17
220µF
R11*
C14*
R14
13k
LGATE1
C21*
FB2 4
14
C22
0.1µF
R12
15k
C6
0.1µF
C5
0.1µF
D2
19
C8
0.1µF
13
C9
1µF
PGOOD 7
VCLK
D4
CS1
BAT254
CPO
20
6
Channel 2 Enable
On
CS2
PGOOD Indicator
3
C7
0.1µF
Channel 1 Enable
5V
FB1
LDO3
D3
R15
20k
BYP1
LDO5
2
D1
R13
10k
Q2
BSC0909
NS
PHASE1
C4*
C18*
9
R10 0
UGATE1
LGATE2
18
10
3.3V Always On
1
R1
82.5k
5
R2
82.5k
C16
1µF
EN1
EN2
GND
21 (Exposed Pad)
Off
* : Optional
VIN
5V to 25V
R8
0
C1
10µF
C10
0.1µF
R4 0 16
Q1
BSC0909
NS
VOUT1
5V
R3 0
L1
3.3µH
C3
220µF
R5*
RT6576B
12
17
C2
0.1µF
Q3
BSC0909
NS
VIN
BOOT2
BOOT1
15
14
C22
0.1µF
R9 0
C11
0.1µF
8
C12
10µF
L2
2.2µH
Q4
BSC0909
NS
11
C13
10µF
VOUT2
3.3V
C17
220µF
R11*
C14*
R14
13k
LGATE1
C21*
C6
0.1µF
C5
0.1µF
D2
19
C8
0.1µF
13
VCLK
D4
C9
1µF
PGOOD 7
CS1
BAT254
CPO
20
6
Channel 2 Enable
On
5V Always On
FB1
CS2
PGOOD Indicator
3
C7
0.1µF
Channel 1 Enable
R15
20k
BYP1
LDO3
3.3V Always On
1
R1
82.5k
5
R2
82.5k
C16
1µF
EN1
EN2
GND
21 (Exposed Pad)
Off
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10
Q2
BSC0909
NS
PHASE1
LDO5
2
D1
D3
* : Optional
9
R10 0
FB2 4
R12
15k
R13
10k
PHASE2
LGATE2
18
10
UGATE1
C4*
C18*
UGATE2
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DS6576A/B-03 April 2022
RT6576A/B
Typical Operating Characteristics
Efficiency vs. Load Current
Efficiency vs. Load Current
100
100
V OUT1
V OUT2
90
VIN
VIN
VIN
VIN
80
=
=
=
=
7.4V
11.1V
14.8V
20V
Efficiency (%)
Efficiency (%)
90
70
60
VIN
VIN
VIN
VIN
80
70
60
EN1 = LDO3, EN2 = 0V, VCLK On
0.01
0.1
1
EN1 = EN2 = LDO3, VCLK On
40
0.001
10
0.01
0.1
Switching Frequency vs. Load Current
250
200
150
100
50
0.01
0.1
1
V OUT2
EN1 = 0V, EN2 = LDO3
VIN = 20V
VIN = 12V
VIN = 7.4V
0
0.001
Switching Frequency vs. Load Current
V OUT1
EN1 = LDO3, EN2 = 0V
300
10
400
Switching Frequency (kHz)1
350
1
Load Current (A)
Load Current (A)
Switching Frequency (kHz)1
7.4V
11.1V
14.8V
20V
50
50
0.001
350
VIN = 20V
VIN = 12V
VIN = 7.4V
300
250
200
150
100
50
0
0.001
10
0.01
0.1
Load Current (A)
1
10
Load Current (A)
Switching Frequency vs. Input Voltage
Switching Frequency vs. Input Voltage
400
350
V OUT2
V OUT1
350
300
Frequency (kHz)1
Switching Frequency (kHz)1
=
=
=
=
250
200
150
100
50
300
250
200
150
100
50
EN1 = 0V, EN2 = LDO3, ILOAD = 6A
EN1 = LDO3, EN2 = 0V, ILOAD = 6A
0
0
5
7
9
11
13
15
17
19
21
23
Input Voltage (V)
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DS6576A/B-03 April 2022
25
5
7
9
11
13
15
17
19
21
23
25
Input Voltage (V)
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RT6576A/B
Output Voltage vs. Load Current
Output Voltage vs. Load Current
4.98
3.335
3.330
4.96
VIN
VIN
VIN
VIN
4.95
=
=
=
=
Output Voltage (V)
Output Voltage (V)
4.97
20V
14.8V
11.1V
7.4V
4.94
4.93
3.325
VIN
VIN
VIN
VIN
3.320
3.315
=
=
=
=
20V
14.8V
11.1V
7.4V
3.310
3.305
3.300
EN1 = LDO3, EN2 = 0V
4.92
0.0001
0.001
0.01
0.1
1
EN1 = 0V, EN2 = LDO3
3.295
0.0001
0.001
0.01
10
Load Current (A)
0.1
1
10
Load Current (A)
LDO5 vs. Load Current
LDO3 vs. Load Current
3.312
5.011
3.311
5.010
3.310
3.309
LDO3 (V)
LDO5 (V)
5.009
5.008
5.007
3.308
3.307
3.306
3.305
5.006
3.304
5.005
3.303
VIN = 12V, EN1 = LDO3, EN2 = 0V, BYP1 Off
VIN = 12V, EN1 = 0V, EN2 = LDO3
3.302
5.004
0
10
20
30
40
50
60
70
80
90
0
100
10
20
40
50
60
70
80
90
100
Load Current (mA)
Load Current (mA)
Quiescent Current vs. Input Voltage
BYP1 Supply Current vs. Input Voltage
500
BYP1 Supply Current (µA)
30
Quiescent Current (µA)
30
25
20
15
10
5
490
480
470
460
450
440
430
420
410
EN1 = EN2 = LDO3, VCLK On, BYP On
0
EN1 = EN2 = LDO3, VCLK On, BYP On
400
5
7
9
11
13
15
17
19
21
23
Input Voltage (V)
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12
25
5
7
9
11
13
15
17
19
21
23
25
Input Voltage (V)
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DS6576A/B-03 April 2022
RT6576A/B
Power On from EN
Power Off from EN
EN
(5V/Div)
EN
(5V/Div)
VOUT2
(3V/Div)
VOUT2
(3V/Div)
VOUT1
(4V/Div)
VOUT1
(4V/Div)
LDO5
(5V/Div)
LDO5
(5V/Div)
VIN = 12V, EN1 = EN2 = LDO3, No Load
VIN = 12V, EN1 = EN2 = LDO3, No Load
Time (2ms/Div)
Time (20ms/Div)
VOUT1 Load Transient Response
VOUT2 Load Transient Response
VOUT1
(100mV/Div)
VOUT2
(100mV/Div)
UGATE1
(50V/Div)
LGATE1
(6V/Div)
UGATE2
(50V/Div)
LGATE2
(6V/Div)
IOUT1
(4A/Div)
IOUT2
(4A/Div)
VIN = 12V, EN1 = 0V,
EN2 = LDO3, IOUT2 = 0A to 6A
VIN = 12V, EN1 = LDO3,
EN2 = 0V, IOUT1 = 0A to 6A
Time (50μs/Div)
Time (50μs/Div)
OVP
UVP
VOUT1
(5V/Div)
VOUT1
(2V/Div)
UGATE1
(20V/Div)
IL1
(4A/Div)
PGOOD
(4V/Div)
LGATE1
(5V/Div)
VIN = 12V, EN1 = EN2 = LDO3, No Load
Time (100μs/Div)
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DS6576A/B-03 April 2022
LGATE1
(10V/Div)
UGATE1
IL1
VIN = 12V, EN1 = EN2 = LDO3
Time (200μs/Div)
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RT6576A/B
Application Information
The RT6576A/B is a dual-channel, low quiescent, Mach
ResponseTM DRVTM mode synchronous Buck controller
targeted for Ultrabook system power supply solutions.
Richtek's Mach ResponseTM technology provides fast
response to load steps. The topology solves the poor load
transient response timing problems of fixed frequency
current mode PWMs, and avoids the problems caused
by widely varying switching frequencies in CCR (constant
current ripple) constant on-time and constant off-time
PWM schemes. A special adaptive on-time control trades
off the performance and efficiency over wide input voltage
range. The RT6576A/B includes 5V (LDO5) and 3.3V
(LDO3) linear regulators. The LDO5 linear regulator steps
down the battery voltage to supply both internal circuitry
and gate drivers. The synchronous switch gate drivers are
directly powered by LDO5. When VOUT1 rises above 4.66V,
an automatic circuit disconnects the linear regulator and
allows the device to be powered by VOUT1 via the BYP1
pin.
on-time is inversely proportional to the input voltage as
measured by VIN and proportional to the output voltage.
The inductor ripple current operating point remains
relatively constant, resulting in easy design methodology
and predictable output voltage ripple. The frequency of 3V
output controller is set higher than the frequency of 5V
output controller. This is done to prevent audio frequency
“ beating” between the two sides, which switch
asynchronously for each side.
The RT6576A/B adaptively changes the operation
frequency according to the input voltage. Higher input
voltage usually comes from an external adapter, so the
RT6576A/B operates with higher frequency to have better
performance. Lower input voltage usually comes from a
battery, so the RT6576A/B operates with lower switching
frequency for lower switching losses. For a specific input
voltage range, the switching cycle period is given by :
For 5V VOUT,
Period (sec.) =
PWM Operation
The Mach ResponseTM DRVTM mode controller relies on
the output filter capacitor's Effective Series Resistance
(ESR) to act as a current sense resistor, so that the output
ripple voltage provides the PWM ramp signal. Referring to
the RT6576A/B's Function Block Diagram, the
synchronous high-side MOSFET is turned on at the
beginning of each cycle. After the internal one-shot timer
expires, the MOSFET will be turned off. The pulse width
of this one-shot is determined by the converter's input
output voltages to keep the frequency fairly constant over
the entire input voltage range. Another one-shot sets a
minimum off-time (200ns typ.). The on-time one-shot will
be triggered if the error comparator is high, the low-side
switch current is below the current limit threshold, and
the minimum off-time one-shot has timed out.
PWM Frequency and On-time Control
For each specific input voltage range, the Mach
ResponseTM control architecture runs with pseudo constant
frequency by feed forwarding the input and output voltage
into the on-time one-shot timer. The high-side switch
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14
VIN 2.7 10-6
VIN 3.79
For 3.3V VOUT,
Period (sec.) =
VIN 2.45 10-6
VIN 2.59
where the VIN is in volt.
The on-time guaranteed in the Electrical Characteristics
table is influenced by switching delays in the external
high-side power MOSFET.
Diode Emulation Mode
In diode emulation mode, the RT6576A/B automatically
reduces switching frequency at light load conditions to
maintain high efficiency. This reduction of frequency is
achieved smoothly. As the output current decreases from
heavy load condition, the inductor current is also reduced,
and eventually comes to the point that its current valley
touches zero, which is the boundary between continuous
conduction and discontinuous conduction modes. To
emulate the behavior of diodes, the low-side MOSFET
allows only partial negative current to flow when the
inductor free wheeling current becomes negative. As the
load current is further decreased, it takes longer and longer
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RT6576A/B
time to discharge the output capacitor to the level that
requires the next “ON” cycle. The on-time is kept the
same as that in the heavy load condition. In reverse, when
the output current increases from light load to heavy load,
the switching frequency increases to the preset value as
the inductor current reaches the continuous conduction.
The transition load point to the light load operation is shown
in Figure 1. and can be calculated as follows :
IL
Slope = (VIN - VOUT) / L
IPEAK
Current Limit Setting
The RT6576A/B has cycle-by-cycle current limit control.
The current limit circuit employs an unique “valley” current
sensing algorithm. If the magnitude of the current sense
signal at PHASEx is above the current limit threshold,
the PWM is not allowed to initiate a new cycle (Figure 2).
The actual peak current is greater than the current limit
threshold by an amount equal to the inductor ripple current.
Therefore, the exact current limit characteristic and
maximum load capability are a function of the sense
resistance, inductor value, battery and output voltage.
IL
ILOAD = IPEAK / 2
IPEAK
ILOAD
0
t
tON
ILIMIT
Figure 1. Boundary Condition of CCM/DEM
(V VOUT )
ILOAD(SKIP) IN
tON
2L
where tON is the on-time.
The switching waveforms may appear noisy and
asynchronous when light load causes diode emulation
operation. This is normal and results in high efficiency.
Trade offs in PFM 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. Penalties for using higher inductor values include
larger physical size and degraded load transient response
(especially at low input voltage levels).
Linear Regulators (LDOx)
The RT6576A/B includes 5V (LDO5) and 3.3V (LDO3)
linear regulators. The regulators can supply up to 100mA
for external loads. Bypass LDOx with a 1μF to 4.7μF, and
recommended value is 1μF ceramic capacitor. When VOUT1
is higher than the switch over threshold (4.66V), an internal
1.5Ω P-MOSFET switch connects BYP1 to the LDO5
pin while simultaneously disconnects the internal linear
regulator.
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t
Figure 2. “Valley” Current Limit
The RT6576A/B uses the on resistance of the synchronous
rectifier as the current sense element and supports
temperature compensated MOSFET RDS(ON) sensing. The
RILIM resistor between the CSx pin and GND sets the current
limit threshold. The resistor RILIM is connected to a current
source from CSx which is 10μA (typ.) at room temperature.
The current source has a 4700ppm/°C temperature slope
to compensate the temperature dependency of the
RDS(ON). When the voltage drop across the sense resistor
or low-side MOSFET equals 1/8 the voltage across the
RILIM resistor, positive current limit will be activated. The
high-side MOSFET will not be turned on until the voltage
drop across the MOSFET falls below 1/8 the voltage across
the RILIM resistor.
Choose a current limit resistor according to the following
equation :
VLIMIT = (RLIMIT x 10μA) − 35mV) / 8 = ILIMIT x RDS(ON)
RLIMIT = ((ILIMIT x RDS(ON)) x 8 + 35mV) / 10μA
Carefully observe the PC board layout guidelines to ensure
that noise and DC errors do not corrupt the current sense
signal at PHASEx and GND. Mount or place the IC close
to the low-side MOSFET.
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RT6576A/B
VCLK for Charge Pump
A 260kHz VCLK signal can be used for the external charge
pump circuit. The VCLK signal becomes available when
EN1 enters ON state. VCLK driver circuit is driven by BYP1
voltage.
The external 14V charge pump is driven by VCLK. As
shown in Figure 3, when VCLK is low, C1 will be charged
by VOUT1 through D1. C1 voltage is equal to VOUT1 minus
the diode drop. When VCLK becomes high, C1 transfers
the charge to C2 through D2 and charges C2 voltage to
VVCLK plus C1 voltage. As VCLK transitions low on the
next cycle, C3 is charged to C2 voltage minus a diode
drop through D3. Finally, C3 charges C4 through D4 when
VCLK switches high. Thus, the total charge pump voltage,
VCP, is :
where VVCLK is the peak voltage of the VCLK driver which
is equal to LDO5 and VD is the forward voltage dropped
across the Schottky diode.
VCLK
C3
VOUT1
Charge Pump
D1
D2
D3
C2
For high current applications, some combinations of high
and low-side MOSFETs may cause excessive gate drain
coupling, which leads to efficiency killing, EMI producing,
and shoot through currents. This is often remedied by
adding a resistor in series with BOOTx, which increases
the turn-on time of the high-side MOSFET without
degrading the turn-off time. See Figure 4.
VIN
UGATEx
BOOTx
RBOOT
PHASEx
Figure 4. Increasing the UGATEx Rise Time
VCP = VOUT1 + 2 x VVCLK − 4 x VD
C1
supply. The instantaneous drive current is supplied by an
input capacitor connected between LDO5 and GND.
D4
C4
Figure 3. Charge Pump Circuit Connected to VCLK
Soft-Start
The RT6576A/B provides an internal soft-start function to
prevent large inrush current and output voltage overshoot
when the converter starts up. The soft-start (SS)
automatically begins once the chip is enabled. During softstart, it clamps the ramping of internal reference voltage
which is compared with FBx signal. The minimum softstart duration is 2ms. A unique PWM duty limit control
that prevents output over-voltage during soft-start period
is designed specifically for FBx floating.
MOSFET Gate Driver (UGATEx, LGATEx)
UVLO Protection
The high-side driver is designed to drive high current, low
RDS(ON) N-MOSFET(s). When configured as a floating driver,
5V bias voltage is delivered from the LDO5 supply. The
average drive current is also calculated by the gate charge
at VGS = 5V times switching frequency. The instantaneous
drive current is supplied by the flying capacitor between
the BOOTx and PHASEx pins. A dead-time to prevent
shoot through is internally generated from high-side
MOSFET off to low-side MOSFET on and low-side
MOSFET off to high-side MOSFET on.
The RT6576A/B has LDO5 under-voltage lock out
protection (UVLO). When the LDO5 voltage is lower than
3.9V (typ.) and the LDO3 voltage is lower than 2.5V (typ.),
both switch power supplies are shut off. This is a nonlatch protection.
The low-side driver is designed to drive high current low
RDS(ON) N-MOSFET(s). The internal pull down transistor
that drives LGATEx low is robust, with a 1Ω typical onresistance. A 5V bias voltage is delivered from the LDO5
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Power Good Output (PGOOD)
PGOOD is an open-drain output and requires a pull-up
resistor. PGOOD is actively held low in soft-start, standby,
and shutdown. For RT6576A/B, PGOOD is released when
both output voltages are above 88% of nominal regulation
point. The PGOOD signal goes low if either output turns
off or is 20% below or 13% over its nominal regulation
point.
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RT6576A/B
Output Over-Voltage Protection (OVP)
Thermal Protection
The output voltage can be continuously monitored for overvoltage condition. If the output voltage exceeds 13% of
its set voltage threshold, the over-voltage protection is
triggered and the LGATEx low-side gate drivers are forced
high. This activates the low-side MOSFET switch, which
rapidly discharges the output capacitor and pulls the output
voltage downward.
The RT6576A/B features thermal shutdown to prevent
damage from excessive heat dissipation. Thermal
shutdown occurs when the die temperature exceeds
150°C. All internal circuitries are turned off during thermal
shutdown. The RT6576A/B triggers thermal shutdown if
LDO5 is not supplied from VOUT1, while input voltage on
VIN and drawing current from LDO5 are too high.
Nevertheless, even if LDO5 is supplied from VOUT1,
overloading LDO5 can cause large power dissipation on
automatic switches, which may still result in thermal
shutdown.
The RT6576A/B is latched once OVP is triggered and can
only be released by either toggling ENx or cycling VIN.
There is a 1μs delay built into the over-voltage protection
circuit to prevent false transition.
Note that latching LGATEx high will cause the output
voltage to dip slightly negative due to previously stored
energy in the LC tank circuit. For loads that cannot tolerate
a negative voltage, place a power Schottky diode across
the output to act as a reverse polarity clamp.
If the over-voltage condition is caused by a shorted in
high-side switch, turning the low-side MOSFET on 100%
will create an electrical shorted circuit between the battery
and GND to blow the fuse and disconnecting the battery
from the output.
Output Under-Voltage Protection (UVP)
The output voltage can be continuously monitored for undervoltage condition. If the output is less than 52% (typ.) of
its set voltage threshold, the under-voltage protection will
be triggered and then both UGATEx and LGATEx gate
drivers will be forced low. The UVP is ignored for at least
4.5ms (typ.) after a start-up or a rising edge on ENx. Toggle
ENx or cycle VIN to reset the UVP fault latch and restart
the controller.
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Discharge Mode (Soft Discharge)
When ENx is low the output under-voltage fault latch is
set, the output discharge mode will be triggered. During
discharge mode, an internal switch creates a path for
discharging the output capacitors' residual charge to GND.
Standby Mode
When VIN exceeds POR threshold and ENx < 0.4V, the
RT6576A/B operate in standby mode, and CH1 and CH2
are OFF state. For the RT6576A, LDO5 is OFF and LDO3
is ON state and approximately consumes 15μA of input
current. For the RT6576B, LDO5 and LDO3 are ON state
and approximately consumes 25μA while in standby mode.
Power-Up Sequencing and On/Off Controls (ENx)
EN1 and EN2 control the power-up sequencing of the two
channels of the Buck converter. The 0.4V falling edge
threshold on ENx can be used to detect a specific analog
voltage level and to shutdown the device. Once in
shutdown, the 1.6V rising edge threshold activates,
providing sufficient hysteresis for most applications.
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RT6576A/B
Table 1. Operation Mode Truth Table
Mode
Condition
Comment
LDO Over
Current Limit
LDOx < UVLO threshold
Transitions to discharge mode after VIN POR. LDO5
and LDO3 remain active.
Run
ENx = high, VOUT1 or VOUT2 are enabled
Normal Operation.
Over-Voltage
Protection
Either output >113% of the nominal level.
LGATEx is forced high. LDO3 and LDO5 are active.
Exit by VIN POR or by toggling ENx.
Under-Voltage
Protection
Either output < 52% of the nominal level Both UGATEx and LGATEx are forced low and enter
after 1.3ms time-out expires and output is discharge mode. LDO3 and LDO5 are active. Exit by
enabled
VIN POR or by toggling ENx.
Discharge
During discharge mode, there is one path to
Either output is still high in standby mode discharge the output capacitors’ residual charge to
GND via an internal switch.
Standby
VIN > POR
ENx < 0.4V
For RT6575A : LDO3 is active
For RT6575B : LDO3, LDO5 are active
Thermal
Shutdown
T J > 150°C
All circuitries are off. Exit by VIN POR.
Table 2. Enabling/PGOOD State (RT6576A)
EN1
EN2
LDO5
LDO3
CH1 (5VOUT)
CH2 (3.3VOUT)
VCLK
PGOOD
OFF
OFF
OFF
ON
OFF
OFF
OFF
Low
ON
OFF
ON
ON
ON
OFF
ON
Low
OFF
ON
ON
ON
OFF
ON
OFF
Low
ON
ON
ON
ON
ON
ON
ON
High
Table 3. Enabling/PGOOD State (RT6576B)
EN1
EN2
LDO5
LDO3
CH1 (5VOUT)
CH2 (3.3VOUT)
VCLK
PGOOD
OFF
OFF
ON
ON
OFF
OFF
OFF
Low
ON
OFF
ON
ON
ON
OFF
ON
Low
OFF
ON
ON
ON
OFF
ON
OFF
Low
ON
ON
ON
ON
ON
ON
ON
High
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RT6576A/B
VIN POR threshold
VIN
LDO3
EN threshold
EN1
VREG5 UVLO threshold
Start-Up Time
LDO5
Soft-Start Time
5V VOUT
EN threshold
EN2
Start-Up Time
3.3V VOUT
PGOOD
Soft-Start Time PGOOD
Delay
Figure 5. RT6576A Timing
VIN POR threshold
VIN
2.5V
LDO3
LDO5
EN threshold
Start-Up Time
EN1
Soft-Start Time
5V VOUT
EN threshold
EN2
Start-Up Time
3.3V VOUT
PGOOD
Soft-Start Time PGOOD
Delay
Figure 6. RT6576B Timing
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RT6576A/B
Output Voltage Setting (FBx)
Output Capacitor Selection
Connect a resistive voltage divider at the FBx pin between
VOUTx and GND to adjust the output voltage from 2V to
5.5V for CH1 and 2V to 4V for CH2, as shown in Figure 7.
The recommended R2 value is between 10kΩ to 20kΩ,
and solve for R1 using the equation below :
R1
VOUT(Valley) VFBx 1 +
R2
where VFBx is 2V (typ.).
The capacitor value and ESR determine the amount of
output voltage ripple and load transient response. Thus,
the capacitor value must be greater than the largest value
calculated from the equations below :
(ILOAD )2 L (tON + tOFF(MIN) )
2 COUT VIN tON VOUTx (tON + tOFF(MIN) )
VSOAR
VIN
UGATEx
VOUTx
PHASEx
LGATEx
VSAG
R1
FBx
R2
GND
(ILOAD )2 L
2 COUT VOUTx
1
VPP LIR ILOAD(MAX) ESR +
8
C
f
OUT
where VSAG and VSOAR are the allowable amount of
undershoot and overshoot voltage during load transient,
Vp-p is the output ripple voltage, and tOFF(MIN) is the
minimum off-time.
Thermal Considerations
Figure 7. Setting VOUTx with a resistive voltage divider
Output Inductor Selection
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as shown
below :
t (VIN VOUTx )
L ON
LIR ILOAD(MAX)
where LIR is the ratio of the peak-to-peak ripple current to
the average inductor current.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice, although powdered iron is
inexpensive and can work well at 200kHz. The core must
be large enough not to saturate at the peak inductor
current, IPEAK :
IPEAK = ILOAD(MAX) + [ (LIR / 2) x ILOAD(MAX) ]
The calculation above shall serve as a general reference.
To further improve transient response, the output
inductance can be further reduced. Of course, besides
the inductor, the output capacitor should also be
considered when improving transient response.
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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, TA is
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-20L 3x3 package, the thermal resistance, θJA, is
30°C/W on a standard JEDEC 51-7 four-layer thermal test
board. The maximum power dissipation at TA = 25°C can
be calculated by the following formula :
P D(MAX) = (125°C − 25°C) / (30°C/W) = 3.33W for
WQFN-20L 3x3 package
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RT6576A/B
Maximum Power Dissipation (W)1
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 8 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
4.0
Layout Considerations
Layout is very important in high frequency switching
converter design. Improper PCB layout can radiate
excessive noise and contribute to the converter’s
instability. Certain points must be considered before
starting a layout with the RT6576A/B.
Four-Layer PCB
3.5
Place the filter capacitor close to the IC, within 12mm
(0.5 inch) if possible.
Keep current limit setting network as close as possible
to the IC. Routing of the network should avoid coupling
to high-voltage switching node.
Connections from the drivers to the respective gate of
the high-side or the low-side MOSFET should be as
short as possible to reduce stray inductance. Use
0.65mm (25 mils) or wider trace.
All sensitive analog traces and components such as
FBx, PGOOD, and should be placed away from high
voltage switching nodes such as PHASEx, LGATEx,
UGATEx, or BOOTx nodes to avoid coupling. Use
internal layer(s) as ground plane(s) and shield the
feedback trace from power traces and components.
Place ground terminal of VIN capacitor(s), V OUTx
capacitor(s), and Source of low-side MOSFETs as close
to each other as possible. The PCB trace of PHASEx
node, which connects to Source of high-side MOSFET,
Drain of low-side MOSFET and high voltage side of the
inductor, should be as short and wide as possible.
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 8. Derating Curve of Maximum Power Dissipation
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RT6576A/B
Outline Dimension
1
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.150
0.250
0.006
0.010
D
2.900
3.100
0.114
0.122
D2
1.650
1.750
0.065
0.069
E
2.900
3.100
0.114
0.122
E2
1.650
1.750
0.065
0.069
e
L
0.400
0.350
0.016
0.450
0.014
0.018
W-Type 20L QFN 3x3 Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
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DS6576A/B-03 April 2022
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