RICHTEK RT9629A

®
RT9629A
Triple-Channel Synchronous Rectified Buck MOSFET
Driver
General Description
Features
The RT9629A is a high frequency, triple-channel
synchronous rectified buck MOSFET driver specifically
designed to drive six power N-MOSFETs. The part is
promoted to pair with Richtek's multiphase buck PWM
controller family for high-density power supply
implementation. The output drivers of RT9629A can
efficiently switch power MOSFETs at frequency 300kHz
typically. Operating in higher frequency should consider
the thermal dissipation carefully. The device implements
bootstrapping on the upper gate with only an external
capacitor and a diode required. This reduces circuit
complexity and allows the use of higher performance, cost
effective N-MOSFETs. All drivers incorporate adaptive
shoot-through protection to prevent upper and lower
MOSFETs from conducting simultaneously and shorting
the input supply. The RT9629A has also detected the fault
condition during initial start-up before the multi-phase PWM
controller takes control. As a result, the input supply will
latch into the shutdown state. The RT9629A comes in a
small footprint package with WQFN-24L 5x5 package.
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Drive Six N-MOSFETs for 3-Phase Buck PWM Control
Shoot Through Protection
Embedded Bootstrap Diode
Support High Switching Frequency
Fast Output Rising Time
Tri-State PWM Input for Output Shutdown
Enable Control
Small 24-Lead WQFN Package
RoHS Compliant and Halogen Free
Applications
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Core Voltage Supplies for Desktop, Motherboard CPU
High Frequency Low Profile DC/DC Converters
High Current Low Voltage DC/DC Converters
Core Voltage Supplies for GFX Card
Marking Information
RT9629AZQW : Product Number
RT9629A
ZQW
YMDNN
YMDNN : Date Code
Simplified Application Circuit
12V
VIN
VCCx
RT9629A
PWM1
PWM1
PWM2
PWM2
PWM3
PWM3
Chip Enable
VOUT
L2
PHASE2
EN1
EN2
EN3
L3
PHASE3
GND
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS9629A-03 October 2012
L1
PHASE1
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT9629A
Ordering Information
Pin Configurations
RT9629A
Note :
Richtek products are :
`
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
LGATE3
VCC2
BOOT2
UGATE2
Lead Plating System
Z : ECO (Ecological Element with
Halogen Free and Pb free)
GND
Package Type
QW : WQFN-24L 5x5 (W-Type)
PHASE3
(TOP VIEW)
24
23
22
21
20
19
UGATE3
1
18
PHASE2
BOOT3
2
17
LGATE2
GND
3
16
VCC1
EN3
4
15
LGATE1
PWM3
5
14
GND
EN2
6
13
PHASE1
GND
10
11
12
BOOT1
9
UGATE1
8
POR
PWM2
7
PWM1
Suitable for use in SnPb or Pb-free soldering processes.
EN1
25
`
WQFN-24L 5x5
Function Pin Description
Pin No.
1, 12, 19
2, 11, 20
Pin Name
UGATE3,
UGATE1,
UGATE2
BOOT3,
BOOT1,
BOOT2
3, 14, 23,
GND
25 (Exposed Pad)
4, 6, 8
5, 7, 9
10
13, 18, 24
15, 17, 22
16, 21
EN3,
EN2,
EN1
PWM3,
PWM2,
PWM1
Pin Function
High Side Gate Drive Outputs for Phase 3, Phase 1, and Phase 2. Connect this
pin to the Gate of high side power MOSFET.
Bootstrap Power Pins for Phase 3, Phase 1, and Phase 2. This pin powers the
high side MOSFET driver. Connect this pin to the junction of the bootstrap
capacitor and the cathode of the bootstrap diode.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
Chip Enable (Active High). When this pin is low, both UGATEx and LGATEx are
driven to low.
PWM Signal Input. Connect this pin to the PWM output of the controller.
POR
Power On Reset Signal.
PHASE1,
PHASE2,
PHASE3
LGATE1,
LGATE2,
LGATE3
Switch Nodes of High Side Driver 1, Driver 2, and Driver 3. Connect this pin to
the high side MOSFET Source together with the low side MOSFET Drain and
the inductor.
VCC1, VCC2
Low Side Gate Drive Output for Phase 1, Phase 2, and Phase 3. This pin drives
the Gate of low side MOSFET.
Supply Input Pin. VCC1 supplies current for Channel 1 and Channel 2 gate
drivers. VCC2 supplies current for Channel 3 gate driver.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS9629A-03 October 2012
RT9629A
Function Block Diagram
VCC1
VCC2
POR
Bootstrap
Control
POR
Enable
Detect
EN1
BOOT1
Internal
VDD
Tri-State
Detect
PWM1
Shoot-Through
Protection
UGATE1
Turn Off
Detection
PHASE1
VCC1
Shoot-Through
Protection
LGATE1
GND
VCC1
Bootstrap
Control
Enable
Detect
EN2
BOOT2
Internal
VDD
Tri-State
Detect
PWM2
Shoot-Through
Protection
UGATE2
Turn Off
Detection
PHASE2
VCC1
Shoot-Through
Protection
LGATE2
GND
VCC2
Bootstrap
Control
Enable
Detect
EN3
Internal
VDD
PWM3
Tri-State
Detect
BOOT3
Shoot-Through
Protection
UGATE3
Turn Off
Detection
PHASE3
VCC2
Shoot-Through
Protection
LGATE3
GND
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS9629A-03 October 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT9629A
Operation
POR (Power On Reset)
Bootstrap Control
POR block detects the voltage at VCC1 pin and VCC2
pin. When the VCC1 and VCC2 pin voltage is higher than
POR rising threshold, POR pin output voltage (POR output)
is high. POR output is low when VCC1 and VCC2 are not
both higher than POR rising threshold. When the POR
pin voltage is high, UGATEx and LGATEx can be controlled
by ENx pin and PWMx pin voltage. With low POR pin
voltage, both UGATEx and LGATEx will be pulled to low.
Bootstrap control block controls the integrated bootstrap
switch. When LGATEx is high (low side MOSFET is
turned on), the bootstrap switch is turned on to charge
the bootstrap capacitor connected to BOOTx pin. When
LGATEx is low (low side MOSFET is turned off), the
bootstrap switch is turned off to disconnect VCCx pin and
BOOTx pin.
Turn-Off Detection
Enable Detect
When ENx pin input voltage is higher/lower than EN rising
threshold, MOSFET driver is enabled/disabled. When the
ENx input and POR output are high, UGATEx and LGATEx
can be controlled by PWMx input voltage. When ENx
input is low, both UGATEx and LGATEx are pulled to low.
Turn-off detection block detects whether high side
MOSFET is turned off by monitoring PHASEx pin voltage.
To avoid shoot through between high side and low side
MOSFETs, low side MOSFET can be turned on only after
high side MOSFET is effectively turned off.
Shoot-Through Protection
Tri-State Detect
When both POR block output and ENx pin voltages are
high, UGATEx and LGATEx can be controlled by PWMx
input. There are three PWMx input modes, which are high,
low, and shutdown state. If PWMx input is within the
shutdown window, both UGATEx and LGATEx output are
low. When PWMx input is higher than its rising threshold,
UGATEx is high and LGATEx is low. When PWMx input
is lower than its falling threshold, UGATEx is low and
LGATEx is high.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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Shoot-through protection block implements the dead time
when both high side and low side MOSFETs are turned
off. With shoot-through protection block, high side and
low side MOSFET are never turned on simultaneously.
Thus, shoot through between high side and low side
MOSFETs is prevented.
is a registered trademark of Richtek Technology Corporation.
DS9629A-03 October 2012
RT9629A
Absolute Maximum Ratings
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(Note 1)
Supply Voltage, VCC1, VCC2 --------------------------------------------------------------------- −0.3V to 15V
BOOTx to PHASEx ---------------------------------------------------------------------------------- −0.3V to 15V
PHASEx to GND
DC -------------------------------------------------------------------------------------------------------- −0.3V to 30V
< 20ns --------------------------------------------------------------------------------------------------- −10V to 35V
LGATEx to GND
DC -------------------------------------------------------------------------------------------------------- −0.3V to (VCC + 0.3V)
< 20ns --------------------------------------------------------------------------------------------------- −2V to (VCC + 0.3V)
UGATEx to GND
DC -------------------------------------------------------------------------------------------------------- (VPHASE − 0.3V) to (VBOOT + 0.3V)
< 20ns --------------------------------------------------------------------------------------------------- (VPHASE − 2V) to (VBOOT + 0.3V)
ENx, PWMx to GND --------------------------------------------------------------------------------- −0.3V to 7V
POR to GND ------------------------------------------------------------------------------------------- −0.3V to 5V
Power Dissipation, PD @ TA = 25°C
WQFN-24L 5x5 --------------------------------------------------------------------------------------- 2.778W
Package Thermal Resistance (Note 2)
WQFN-24L 5x5, θJA ---------------------------------------------------------------------------------- 36°C/W
WQFN-24L 5x5, θJC --------------------------------------------------------------------------------- 6°C/W
Lead Temperature (Soldering, 10 sec.) ---------------------------------------------------------- 260°C
Junction Temperature -------------------------------------------------------------------------------- 150°C
Storage Temperature Range ----------------------------------------------------------------------- −65°C to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) ------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
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(Note 4)
Supply Voltage, VCC1, VCC2 --------------------------------------------------------------------- 4.5V to 13.2V
Junction Temperature Range ----------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range ----------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VCCx = 12V, TA = 25°C unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
4.5
--
13.2
V
Power Supply Voltage
VCC
Power Supply Current
IVCC
VBOOTx = 12V, PWMx Floating
--
250
--
μA
POR Rising Threshold
VPOR_r
VCCx Rising
--
4
4.4
V
POR Falling Threshold
VPOR_f
VCCx Falling
3
3.5
--
V
POR Pin High Voltage
VPOR_H
--
3.5
4
V
POR Pin Low Voltage
VPOR_L
--
--
0.5
V
Power On Reset (POR)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS9629A-03 October 2012
is a registered trademark of Richtek Technology Corporation.
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5
RT9629A
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
EN Input
ENx Rising Threshold
VENH
--
1.3
1.6
V
ENx Falling Threshold
VENL
0.7
1
--
V
PWM Input
Maximum Input Current
IPWM
VPWMx = 0V or 5V
--
160
--
μA
PWMx Floating Voltage
VPWM_fl
PWMx = Open
--
1.8
--
V
PWMx Rising Threshold
VPWM_rth
2.3
2.8
3.2
V
PWMx Falling Threshold VPWM_fth
0.7
1.1
1.4
V
Timing
UGATEx Rising Time
tUGATEr
3nF load
--
25
--
ns
UGATEx Falling Time
tUGATEf
3nF load
--
12
--
ns
LGATEx Rising Time
tLGATEr
3nF load
--
24
--
ns
LGATEx Falling Time
tLGATEf
3nF load
--
10
--
ns
Propagation Delay
tUGATEpgh VBOOTx − VPHASEx = 12V
tUGATEpdl See Timing Diagram
--
30
--
--
22
--
tLGATEpdh
--
30
--
--
8
--
tLGATEpdl
See Timing Diagram
ns
ns
Output
UGATEx Drive Source
RUGATEsr
VBOOT − VPHASE = 12V, ISource = 100mA
--
1.7
--
Ω
UGATEx Drive Sink
RUGATEsk
VBOOT − VPHASE = 12V, ISink = 100mA
--
1.4
--
Ω
LGATEx Drive Source
RLGATEsr
ISource = 100mA
--
1.6
--
Ω
LGATEx Drive Sink
RLGATEsk
ISink = 100mA
--
1.1
--
Ω
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 precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS9629A-03 October 2012
RT9629A
Typical Application Circuit
RT9629A
16 VCC1
RVCC
2.2
12V
21 VCC2
RBOOT1
1
CBOOT1
11
BOOT1
1µF R
UG1
2.2
12
UGATE1
CVCC
1µF
PHASE1
10
8
Chip Enable
6
4
9
PWM1
LGATE1
POR
L1
15
RLG1
0
EN2
EN3
PHASE2 18
PWM1
PWM2
7 PWM2
PWM3
5 PWM3
3, 14, 23,
GND
25 (Exposed Pad)
LGATE2
QUG1
13
RBOOT2
1
CBOOT2
BOOT2 20
1µF R
UG2
2.2
19
UGATE2
EN1
VIN
12V
CIN
270µF x 3
17
RPH1
2.2
QLG1
CPH1
3.3nF
VIN
QUG2
L2
RLG2
0
RBOOT3
1
CBOOT3
2
BOOT3
1µF R
UG3
2.2
1
UGATE3
RPH2
2.2
QLG2
CPH2
3.3nF
VIN
QUG3
L3
PHASE3 24
LGATE3
22
VOUT
COUT
820µF x 6
RLG3
0
RPH3
2.2
QLG3
CPH3
3.3nF
Timing Diagram
PWMx
tLGATEpdl
LGATEx
90%
tUGATEpdl
1.5V
1.5V
1.5V
90%
1.5V
UGATEx
tUGATEpdh
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS9629A-03 October 2012
tLGATEpdh
is a registered trademark of Richtek Technology Corporation.
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7
RT9629A
Typical Operating Characteristics
Drive Enable
Drive Disable
UGATE
(50V/Div)
UGATE
(50V/Div)
PHASE
(20V/Div)
PHASE
(20V/Div)
LGATE
(20V/Div)
LGATE
(20V/Div)
EN
(10V/Div)
EN
(10V/Div)
VIN = 12V, No Load
VIN = 12V, No Load
Time (1μs/Div)
Time (1μs/Div)
PWM Rising Edge
PWM Falling Edge
PWM
(10V/Div)
PWM
(10V/Div)
UGATE
(20V/Div)
UGATE
(20V/Div)
LGATE
(10V/Div)
LGATE
(10V/Div)
PHASE
(10V/Div)
PHASE
(10V/Div)
Time (20ns/Div)
Time (20ns/Div)
Dead Time
Dead Time
UGATE
UGATE
PHASE
PHASE
LGATE
LGATE
(5V/Div)
(5V/Div)
Full Load
Full Load
Time (20ns/Div)
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Time (20ns/Div)
is a registered trademark of Richtek Technology Corporation.
DS9629A-03 October 2012
RT9629A
Dead Time
Dead Time
UGATE
UGATE
PHASE
PHASE
LGATE
(5V/Div)
LGATE
(5V/Div)
No Load
Time (20ns/Div)
No Load
Time (20ns/Div)
Short Pulse
UGATE
LGATE
PHASE
(5V/Div)
UGATE − PHASE
No Load
Time (20ns/Div)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS9629A-03 October 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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RT9629A
Application Information
The RT9629A is a high frequency, triple-channel
synchronous rectified. MOSFET driver containing
Richtek's advanced MOSFET driver technologies. The
RT9629A is designed to be able to adapt from normal
MOSFET driving applications to high performance CPU
VR driving capabilities.
Supply Voltage and Power On Reset
The RT9629A can be utilized under both VCCx = 5V or
VCCx = 12V applications which may happen in different
fields of electronics application circuits. In terms of
efficiency, higher VCCx equals higher driving voltage of
UGATEx/LGATEx which may result in higher switching
loss and lower conduction loss of power MOSFETs. The
choice of VCCx = 12V or VCCx = 5V can be a tradeoff to
optimize system efficiency. And VCC1 pin must be directly
connected to VCC2 pin.
The RT9629A controls both high side and low side NMOSFETs of three half-bridge power according to three
external input PWMx control signals. It has Power On
Reset (POR) function which held UGATEx and LGATEx
low before the VCCx voltage rises to higher than rising
threshold voltage. When VCC1 and VCC2 exceed the POR
threshold voltage, the voltage at the POR pin will be pulled
high.
Enable and Disable
The RT9629A includes an ENx pin for sequence control.
When the ENx pin rises above the VENH trip point, the
RT9629A begins a new initialization and follows the PWMx
command to control the UGATEx and LGATEx. When the
ENx pin falls below the VENL trip point, the RT9629A shuts
down and keeps UGATEx and LGATEx low.
The PWMx signal is acted as “ High” if the signal is above
the rising threshold and acted as “ Low” if the signal is
below the falling threshold. When PWM signal level enters
and remains within the shutdown window, the output drivers
are disabled and both MOSFET gates are pulled and held
low. If the PWMx signal is left floating, the pin will be kept
around 1.8V by the internal divider and provide the PWMx
controller with a recognizable level.
Bootstrap Power Switch
The RT9629A builds in an internal bootstrap power switch
to replace external bootstrap diode, and this can facilitate
PCB design and reduce total BOM cost of the system.
Hence, no external bootstrap diode is required in real
applications.
Non-overlap Control
To prevent the overlap of the gate drivers during the
UGATEx pull low and the LGATEx pull high, the non-overlap
circuit monitors the voltages at the PHASEx node and
high side gate drive (UGATEx − PHASEx). When the
PWMx input signal goes low, UGATEx begins to pull low
(after propagation delay). Before LGATEx is pulled high,
the non-overlap protection circuit ensures that the
monitored voltages have gone below 1.1V. Once the
monitored voltages fall below 1.1V, LGATEx begins to turn
high. By waiting for the voltages of the PHASEx pin and
high side gate driver to fall below 1.1V, the non-overlap
protection circuit ensures that UGATEx is low before
LGATEx pulls high.
Also to prevent the overlap of the gate drivers during
LGATEx pull low and UGATEx pull high, the non-overlap
circuit monitors the LGATEx voltage. When LGATEx goes
below 1.1V, UGATEx goes high after propagation delay.
Tri-state PWM Input
After the initialization, the PWMx signal takes the control.
The rising PWMx signal first forces the LGATEx signal to
turn low then UGATEx signal is allowed to go high just
after a non-overlapping time to avoid shoot through current.
The falling of PWMx signal first forces UGATEx to go low.
When UGATEx and PHASEx signal reach a
predetermined low level, LGATEx signal is allowed to turn
high.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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10
Driving Power MOSFETs
The DC input impedance of the power MOSFET is
extremely high. When Vgs1 or Vgs2 is at 12V or 5V, the
gate draws the current only for few nano-amperes. Thus
once the gate has been driven up to “ ON” level, the
current could be negligible.
is a registered trademark of Richtek Technology Corporation.
DS9629A-03 October 2012
RT9629A
However, the capacitance at the gate to source terminal
should be considered. It requires relatively large currents
to drive the gate up and down 12V (or 5V) rapidly. It is
also required to switch drain current on and off with the
required speed. The required gate drive currents are
calculated as follows.
d1
s1
VPHASEx
VIN
L
VOUT
Cgs1
Cgd1
Cgd2
Igs1
Igd1
Ig1
g1
d2
D2
Igs2
Cgs2
dVg2
dt
=
Cgs1 x 12
(2)
tr2
Before driving the gate of the high side MOSFET up to
12V, the low side MOSFET has to be off; and the high
side MOSFET will be turned off before the
low side is turned on. From Figure 1, the body diode “ D2”
will be turned on before high side MOSFETs turn on.
dV
12
Igd1 = Cgd1
= Cgd1
(3)
dt
tr1
Before the low side MOSFET is turned on, the Cgd2 have
been charged to VIN. Thus, as Cgd2 reverses its polarity
and g2 is charged up to 12V, the required current is
Ig2 Igd2
g2
Igs2 = Cgs1
Igd2 = Cgd2
s2
VIN + 12
dV
= Cgd2
dt
tr2
(4)
GND
It is helpful to calculate these currents in a typical case.
Assume a synchronous rectified Buck converter, input
voltage VIN = 12V, Vgs1 = 12V, Vgs2 = 12V.The high side
MOSFET is PHB83N03LT whose C iss = 1660pF,
Crss = 380pF, and tr = 14ns. The low side MOSFET is
PHB95N03LT whose Ciss = 2200pF, Crss = 500pF and
tr = 30ns, from the equation (1) and (2) we can obtain
Vg1
VPHASEx +12V
t
Vg2
12V
Igs1 =
t
Figure 1. Equivalent Circuit and Waveforms (VCC = 12V)
In Figure 1, the current Ig1 and Ig2 are required to move the
gate up to 12V. The operation consists of charging Cgd1,
Cgd2 , Cgs1 and Cgs2. Cgs1 and Cgs2 are the capacitors from
gate to source of the high side and the low side power
MOSFETs, respectively. In general data sheets, the Cgs1
and C gs2 are referred as “ Ciss” which are the input
capacitors. Cgd1 and Cgd2 are the capacitors from gate to
drain of the high side and the low side power MOSFETs,
respectively and referred to the data sheets as “ Crss” the
reverse transfer capacitance. For example, tr1 and tr2 are
the rising time of the high side and the low side power
MOSFETs respectively, the required current Igs1 and Igs2,
are shown as below :
Igs1 = Cgs1
dVg1
dt
=
Cgs1 x 12
tr1
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS9629A-03 October 2012
Igs2 =
1660 x 10-12 x 12
14 x 10-9
2200 x 10-12 x 12
30 x 10-9
= 1.428
= 0.88
(5)
(A)
(A)
(6)
from equation. (3) and (4)
Igd1 =
Igd2 =
380 x 10-12 x 12
14 x 10-9
= 0.326 (A)
500 x 10-12 x (12+12 )
30 x 10-9
(7)
= 0.4 (A)
(8)
the total current required from the gate driving source can
be calculated as following equations.
Ig1 = Igs1 + Igd1 = (1.428 + 0.326 ) = 1.754 (A)
Ig2 = Igs2 + Igd2 = ( 0.88 + 0.4 ) = 1.28 (A)
(9)
(10)
By a similar calculation, we can also get the sink current
required from the turned off MOSFET.
(1)
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11
RT9629A
Select the Bootstrap Capacitor
Figure 2 shows part of the bootstrap circuit of the
RT9629A. The VCB (the voltage difference between BOOTx
and PHASEx on RT9629A) provides a voltage to the gate
of the high side power MOSFET. This supply needs to be
ensured that the MOSFET can be driven. For this, the
capacitance CBOOT has to be selected properly. It is
determined by the following constraints.
CBOOT
1µF
10
12V
1µF
12V
BOOTx
VCCx UGATEx
2N7002
RT9629A
POR
Chip Enable
PWMx
CU
3nF
POR
PHASEx
ENx
PWNx
VIN
2N7002
20
LGATEx
GND
CL
3nF
BOOTx
UGATEx
PHASEx
CBOOT
Figure 3. Power Dissipation Test Circuit
+
VCB
-
VCCx
LGATEx
Figure 4 shows the power dissipation of the RT9629A as
a function of frequency and load capacitance when VCC =
12V. The value of CU and CL are the same and the frequency
is varied from 100kHz to 1MHz.
Power Dissipation vs. Frequency
GND
1000
In practice, a low value capacitor CBOOT will lead to the
over charging that could damage the IC. Therefore, to
minimize the risk of overcharging and to reduce the ripple
on VCB, the bootstrap capacitor should not be smaller than
0.1μF, and the larger the better. In general design, using
1μF can provide better performance. At least one low-ESR
capacitor should be used to provide good local de-coupling.
It is recommended to adopt a ceramic or tantalum
capacitor.
Power Dissipation
To prevent driving the IC beyond the maximum
recommended operating junction temperature of 125°C,
it is necessary to calculate the power dissipation
appropriately. This dissipation is a function of switching
frequency and total gate charge of the selected MOSFET.
Figure 3 shows the power dissipation test circuit. CL and
C U are the UGATEx and LGATEx load capacitors,
respectively. The bootstrap capacitor value is 1μF.
Power Dissipation (mW)
900
Figure 2. Part of Bootstrap Circuit of RT9629A
800
CU = CL = 3nF
700
600
CU = CL = 2nF
500
400
300
200
CU = CL = 1nF
100
VCC = 12V
0
0
200
400
600
800
1000
Frequency (kHz)
Figure 4. Power Dissipation vs. Frequency
The operating junction temperature can be calculated from
the power dissipation curves (Figure 4). Assume VCCx =
12V, operating frequency is 200kHz and CU = CL = 1nF
which emulate the input capacitances of the high side
and low side power MOSFETs. From Figure 4, the power
dissipation is 100mW. Thus, for example, with the SOP8 package, the package thermal resistance θJA is 120°C/
W. The operating junction temperature is then calculated
as :
TJ = (120°C/W x 100mW) + 25°C = 37°C
(11)
where the ambient temperature is 25°C.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
12
is a registered trademark of Richtek Technology Corporation.
DS9629A-03 October 2012
RT9629A
Thermal Considerations
Layout Consideration
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 :
Figure 6 shows the schematic circuit of a synchronous
buck converter to implement the RT9629A. The converter
operates from 5V to 12V of input Voltage.
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-24L 5x5 package, the thermal resistance, θJA, is
36°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 :
For the PCB layout, it should be very careful. The power
circuit section is the most critical one. If not configured
properly, it will generate a large amount of EMI. The location
of QUGx, QLGx, Lx should be very close.
Next, the trace from UGATEx, and LGATEx should also
be short to decrease the noise of the driver output signals.
PHASEx signals from the junction of the power MOSFET,
carrying the large gate drive current pulses, should be as
heavy as the gate drive trace. The bypass capacitor CVCC
should be connected to GND directly. Furthermore, the
bootstrap capacitors (CBOOTx) should always be placed as
close to the pins of the IC as possible.
VIN
12V
LIN
12V
+
PD(MAX) = (125°C − 25°C) / (36°C/W) = 2.778W for
CIN
CIN2
CBOOTx
RVCC
BOOTx
VCCx
WQFN-24L 5x5 package
Maximum Power Dissipation (W)1
3.0
Four-Layer PCB
UGATEx
PHB83N03LT
+
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 5 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
PHASEx
COUT
QLGx
PHB95N03LT LGATEx
RT9629A
CVCC
QUGx
Lx
VOUT
PWMx
ENx
PWMx
ENx
GND
Figure 6. Synchronous Buck Converter Circuit
2.5
2.0
1.5
1.0
0.5
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 5. Derating Curve of Maximum Power Dissipation
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS9629A-03 October 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT9629A
Outline Dimension
D2
D
SEE DETAIL A
L
1
E
E2
e
1
2
2
DETAIL A
Pin #1 ID and Tie Bar Mark Options
b
A
A3
1
Note : The configuration of the Pin #1 identifier is optional,
but must be located within the zone indicated.
A1
Dimensions In Millimeters
Dimensions In Inches
Symbol
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
0.250
0.350
0.010
0.014
D
4.950
5.050
0.195
0.199
D2
3.100
3.400
0.122
0.134
E
4.950
5.050
0.195
0.199
E2
3.100
3.400
0.122
0.134
e
L
0.650
0.350
0.026
0.450
0.014
0.018
W-Type 24L QFN 5x5 Package
Richtek Technology Corporation
5F, No. 20, Taiyuen 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.
www.richtek.com
14
DS9629A-03 October 2012