LTC4447
High Speed Synchronous
N-Channel MOSFET Driver
FEATURES
■
■
■
■
■
■
■
■
■
■
■
■
■
DESCRIPTION
Integrated Schottky Diode
4V to 6.5V VCC Operating Voltage
38V Maximum Input Supply Voltage
Adaptive Shoot-Through Protection
Rail-to-Rail Output Drivers
3.2A Peak Pull-Up Current
4.5A Peak Pull-Down Current
8ns TG Risetime Driving 3000pF Load
7ns TG Falltime Driving 3000pF Load
Separate Supply to Match PWM Controller
Drives Dual N-Channel MOSFETs
Undervoltage Lockout
Low Profile (0.75mm) 3mm × 3mm DFN Package
APPLICATIONS
■
■
Distributed Power Architectures
High Density Power Modules
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All
other trademarks are the property of their respective owners.
The LTC®4447 is a high frequency gate driver with integrated bootstrap Schottky diode that is designed to drive two
N-Channel MOSFETs in a synchronous DC/DC converter.
The powerful rail-to-rail driver capability reduces switching
losses in MOSFETs with high gate capacitance.
The LTC4447 features a separate supply for the input logic
to match the signal swing of the controller IC. If the input
signal is not being driven, the LTC4447 activates a shutdown mode that turns off both external MOSFETs. The input
logic signal is internally level-shifted to the bootstrapped
supply, which functions at up to 42V above ground. The
Schottky diode required for the bootstrapped supply is
integrated to simplify layout and reduce parts count.
The LTC4447 contains undervoltage lockout circuits on
both the driver and logic supplies that turn off the external
MOSFETs when an undervoltage condition is present. An
adaptive shoot-through protection feature is also built-in
to prevent the power loss resulting from MOSFET crossconduction current.
The LTC4447 is available in the 3mm × 3mm DFN
package.
TYPICAL APPLICATION
Synchronous Buck Converter Driver
LTC4447 Driving 3000pF Capacitive Loads
VCC
4V TO 6.5V
VCC
BOOST
VLOGIC
LTC4447
IN
GND
TOP GATE
(TG - TS)
5V/DIV
TG
TS
PWM
INPUT (IN)
5V/DIV
VIN
TO 38V
VOUT
BOTTOM GATE
(BG) 5V/DIV
BG
4447 TA01a
10ns/DIV
4447 TA01b
4447f
1
LTC4447
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
Supply Voltage
VLOGIC ...................................................... –0.3V to 7V
VCC........................................................... –0.3V to 7V
BOOST – TS ............................................. –0.3V to 7V
BOOST Voltage .......................................... –0.3V to 42V
BOOST – VCC ............................................................38V
TS + VCC....................................................................42V
IN Voltage .................................................... –0.3V to 7V
Driver Output TG (with Respect to TS)......... –0.3V to 7V
Driver Output BG.......................................... –0.3V to 7V
Operating Temperature Range (Note 2).... –40°C to 85°C
Junction Temperature (Note 3) ............................. 125°C
Storage Temperature Range................... –65°C to 150°C
TOP VIEW
NC
NC
TG
TS
BG
GND
12 BOOST
11 BOOST
10 BOOST
9 VCC
8 VLOGIC
7 IN
1
2
3
13
4
5
6
DDMA PACKAGE
12-LEAD (3mm ´ 3mm) PLASTIC DFN
θJA = 43°C/W, θJC = 3°C/W
EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC4447EDD#PBF
LTC4447EDD#TRPBF
LDHD
12-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
LTC4447IDD#PBF
LTC4447IDD#TRPBF
LDHD
12-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *Temperature grades are identified by a label on the shipping container.
Consult LTC Marketing for information on lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = VLOGIC = 5V, VTS = GND = 0V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Logic Supply (VLOGIC)
VLOGIC
Operating Range
3
IVLOGIC
DC Supply Current
IN = Floating
UVLO
Undervoltage Lockout Threshold
VLOGIC Rising
VLOGIC Falling
Hysteresis
●
●
2.5
2.4
6.5
V
730
900
μA
2.75
2.65
100
3
2.9
V
V
mV
6.5
V
600
800
μA
3.20
3.04
160
3.65
3.50
V
V
mV
Gate Driver Supply (VCC)
VCC
Operating Range
4
IVCC
DC Supply Current
IN = Floating
UVLO
Undervoltage Lockout Threshold
VCC Rising
VCC Falling
Hysteresis
VD
Schottky Diode Forward Voltage
ID = 10mA
ID = 100mA
●
●
2.75
2.60
0.38
0.48
V
V
4447f
2
LTC4447
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = VLOGIC = 5V, VTS = GND = 0V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
VIH(TG)
TG Turn-On Input Threshold
VLOGIC ≥ 5V, IN Rising
VLOGIC = 3.3V, IN Rising
VIL(TG)
TG Turn-Off Input Threshold
VIH(BG)
UNITS
●
●
3
1.9
3.5
2.2
4
2.6
V
V
VLOGIC ≥ 5V, IN Falling
VLOGIC = 3.3V, IN Falling
●
●
2.75
1.8
3.25
2.09
3.75
2.5
V
V
BG Turn-On Input Threshold
VLOGIC ≥ 5V, IN Falling
VLOGIC = 3.3V, IN Falling
●
●
0.8
0.8
1.25
1.1
1.6
1.4
V
V
VIL(BG)
BG Turn-Off Input Theshold
VLOGIC ≥ 5V, IN Rising
VLOGIC = 3.3V, IN Rising
●
●
1.05
0.9
1.5
1.21
1.85
1.5
V
V
IIN(SD)
Maximum Current Into or Out of IN in
Shutdown Mode
VLOGIC ≥ 5V, IN Floating
VLOGIC = 3.3V, IN Floating
150
75
300
150
Input Signal (IN)
μA
μA
High Side Gate Driver Output (TG)
VOH(TG)
TG High Output Voltage
ITG = –100mA, VOH(TG) = VBOOST – V TG
140
mV
VOL(TG)
TG Low Output Voltage
ITG = 100mA, VOL(TG) = V TG – V TS
80
mV
IPU(TG)
TG Peak Pull-Up Current
●
2
3.2
A
IPD(TG)
TG Peak Pull-Down Current
●
1.5
2.4
A
Low Side Gate Driver Output (BG)
VOH(BG)
BG High Output Voltage
IBG = –100mA, VOH(BG) = VCC – VBG
100
mV
VOL(BG)
BG Low Output Voltage
IBG = 100mA
100
mV
IPU(BG)
BG Peak Pull-Up Current
●
2
3.2
A
IPD(BG)
BG Peak Pull-Down Current
●
3
4.5
A
Switching Time
tPLH(TG)
BG Low to TG High Propagation Delay
14
ns
tPHL(TG)
IN Low toTG Low Propagation Delay
13
ns
tPLH(BG)
TG Low to BG High Propagation Delay
13
ns
tPHL(BG)
IN High to BG Low Propagation Delay
11
ns
tr(TG)
TG Output Risetime
10% to 90%, CL = 3nF
8
ns
t f(TG)
TG Output Falltime
10% to 90%, CL = 3nF
7
ns
tr(BG)
BG Output Risetime
10% to 90%, CL = 3nF
7
ns
t f(BG)
BG Output Falltime
10% to 90%, CL = 3nF
4
ns
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC4447I is guaranteed to meet specifications from –40°C
to 85°C. The LTC4447E is guaranteed to meet specifications from 0°C
to 85°C with specifications over the –40°C to 85°C temperature range
assured by design, characterization and correlation with statistical
process controls.
TJ is calculated from the ambient temperature TA and power dissipation
PD according to the following formula:
TJ = TA + (PD • 43°C/W)
Note 3: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is
active. Continuous operation above the specified maximum operating
junction temperature may impair device reliability.
4447f
3
LTC4447
TYPICAL PERFORMANCE CHARACTERISTICS
Input Thresholds vs VLOGIC Supply
Voltage
Input Thresholds for VLOGIC ≥ 5V
vs Temperature
Input Thresholds for VLOGIC = 3.3V
vs Temperature
4.0
3.0
VIH(TG)
5
VLOGIC = 3.3V
VLOGIC ≥ 5V
2.5
2.0
VIL(BG)
1.5
VIH(BG)
1.0
2.0
VIL(TG)
1.5
VIL(BG)
1.0
3.5
4.0 4.5 5.0
5.5
VLOGIC SUPPLY (V)
6.0
0.5
–40
6.5
–10
20
50
80
TEMPERATURE (°C)
0.25
0.20
0.15
0.10
0.05
0
3.0
3.5
4.0 4.5 5.0
5.5
VLOGIC SUPPLY (V)
6.0
6.5
0.35
IN FLOATING
0.9 TS = GND
VLOGIC = 5V
0.25
0.20
0.15
VLOGIC = 3.3V
0.10
VCC UVLO THRESHOLD (V)
VLOGIC UVLO THRESHOLD (V)
2.5
–40
–10
20
80
50
TEMPERATURE (°C)
0.7
0.6
110
4447 G08
IVCC
0.5
0.4
0.3
0.2
0.05
0.1
0
–40
–10
20
50
80
TEMPERATURE (°C)
0
110
3.0 3.5
4.0 4.5 5.0 5.5 6.0
SUPPLY VOLTAGE (V)
7.0
Undervoltage Lockout Threshold
Hysteresis vs Temperature
250
3.2
RISING THRESHOLD
3.1
FALLING THRESHOLD
3.0
2.9
–40
6.5
4447 G06
3.3
2.6
110
IVLOGIC
0.8
0.30
4447 G05
2.9
FALLING THRESHOLD
20
80
50
TEMPERATURE (°C)
Quiescent Supply Current vs
Supply Voltage
VCC Undervoltage Lockout
Thresholds vs Temperature
2.7
–10
1.0
VLOGIC Undervoltage Lockout
Thresholds vs Temperature
RISING THRESHOLD
VIH(BG)
0.40
4447 G04
2.8
VIL(BG)
4447 G03
BG or TG Input Threshold Hysteresis
vs Temperature
BG OR TG INPUT THRESHOLD HYSTERESIS (V)
BG OR TG INPUT THRESHOLD HYSTERESIS (V)
BG or TG Input Threshold Hysteresis
vs VLOGIC Supply Voltage
0.30
2
4447 G02
4447 G01
0.35
VIL(TG)
3
0
–40
110
UVLO THRESHOLD HYSTERESIS (V)
3.0
VIH(TG)
1
VIH(BG)
0.5
0
INPUT THRESHOLD (V)
2.5
4
VIH(TG)
SUPPLY CURRENT (mA)
VIL(TG)
3.0
INPUT THRESHOLD (V)
INPUT THRESHOLD (V)
3.5
–10
20
80
50
TEMPERATURE (°C)
110
4447 G09a
200
VCC UVLO
150
100
VLOGIC UVLO
50
0
–40
–10
20
80
50
TEMPERATURE (°C)
110
4447 G09b
4447f
4
LTC4447
TYPICAL PERFORMANCE CHARACTERISTICS
Schottky Diode Forward Voltage
vs Diode Current
Schottky Diode Forward Voltage
vs Temperature
6
0.6
0.5
0.4
0.3
0.2
0.1
0
50
100
150
DIODE CURRENT (mA)
ID = 100mA
0.4
ID = 10mA
0.3
ID = 1mA
0.2
0
–40
200
RISE/FALL TIME (ns)
ILOGIC
fIN = 500kHz
600k
1M
800k
Rise and Fall Time vs Load
Capacitance
100
VCC = 5V
TS = GND
tr(TG)
10
tf(TG)
tr(TG)
tr(BG)
5
tf(TG)
10
tr(BG)
tf(BG)
tf(BG)
3
10
LOAD CAPACITANCE (nF)
1
0
3.5
30
1
4.0
5.5
5.0
6.0
4.5
VCC SUPPLY VOLTAGE (V)
4447 G13
Propagation Delay vs
VCC Supply Voltage
25
PROPAGATION DLEAY (ns)
tpLH(BG)
15
tpHL(TG)
10
tpHL(BG)
3.5
25
NO LOAD
VLOGIC = 5V
TS = GND
tpLH(TG)
5
3.0
Propagation Delay vs
Temperature
20
NO LOAD
VCC = 5V
TS = GND
5.0 5.5 6.0
4.5
VLOGIC SUPPLY VOLTAGE (V)
4.0
6.5
4447 G16
30
4447 G15
4447 G14
Propagation Delay vs VLOGIC
Supply Voltage
20
10
3
LOAD CAPACITANCE (nF)
1
6.5
15
tpLH(TG)
PROPAGATION DELAY (ns)
SUPPLY CURRENT (mA)
ICC
fIN = 100kHz
400k
4447 G12
CLOAD = 3.3nF
TS = GND
ICC
fIN = 500kHz
200k
FREQUENCY (Hz)
15
0.1
PROPAGATION DLEAY (ns)
IVLOGIC
0
110
Rise and Fall Time vs VCC Supply
Voltage
VLOGIC = VCC = 5V
TS = GND
1
2
4447 G11
Switching Supply Current vs Load
Capacitance
10
IVCC
3
0
20
80
50
TEMPERATURE (°C)
–10
4447 G10
100
4
1
0.1
RISE/FALL TIME (ns)
0
0.5
NO LOAD
VLOGIC = VCC = 5V
5 TS = GND
SUPPLY CURRENT (mA)
SCHOTTKY DIODE FORWARD VOLTAGE (V)
DIODE FORWARD VOLTAGE (V)
0.6
Supply Current vs Input
Frequency
tpLH(BG)
tpHL(TG)
10
tpHL(BG)
20
15
NO LOAD
VCC = VLOGIC = 5V
TS = GND
tpHL(TG)
tpLH(TG)
10
tpHL(BG)
tpLH(BG)
5
5
4.0
5.5
5.0
6.0
4.5
VCC SUPPLY VOLTAGE (V)
6.5
4447 G17
0
–40
–10
20
50
80
TEMPERATURE (°C)
110
4447 G18
4447f
5
LTC4447
PIN FUNCTIONS
NC (Pins 1, 2): No Connection Required.
TG (Pin 3): High Side Gate Driver Output (Top Gate). This
pin swings between TS and BOOST.
TS (Pin 4): High Side MOSFET Source Connection (Top
Source).
BG (Pin 5): Low Side Gate Driver Output (Bottom Gate).
This pin swings between VCC and GND.
GND (Pin 6): Chip Ground.
IN (Pin 7): Input Signal. Input referenced to an internal
supply baised off of VLOGIC (Pin 8) and GND (Pin 6). If
this pin is floating, an internal resistive divider triggers a
shutdown mode in which both BG (Pin 5) and TG (Pin 3)
are pulled low. Trace capacitance on this pin should be
minimized to keep the shutdown time low.
VLOGIC (Pin 8): Logic Supply. This pin powers the input
buffer and logic. Connect this pin to the power supply
of the controller that is driving IN (Pin 7) to match input
thresholds or to VCC (Pin 9) to simplify PCB routing.
VCC (Pin 9): Output Driver Supply. This pin powers the
low side gate driver output directly and the high side gate
driver output through an internal Schottky diode connected between this pin and BOOST. A low ESR ceramic
bypass capacitor should be tied between this pin and
GND (Pin 6).
BOOST (Pins 10, 11, 12): High Side Bootstrapped Supply.
An external capacitor should be tied between these pins
and TS (Pin 4). An internal Schottky diode is connected
between VCC (Pin 9) and these pins. Voltage swing at these
pins is from VCC – VD to VIN + VCC – VD, where VD is the
forward voltage drop of the Schottky diode.
Exposed Pad (Pin 13): Ground. The exposed pad must be
soldered to PCB ground for optimal electrical and thermal
performance.
BLOCK DIAGRAM
9
VCC
12
UNDERVOLTAGE
LOCKOUT
11
BOOST
10
8
VLOGIC
UNDERVOLTAGE
LOCKOUT
TG
LEVEL
SHIFTER
TS
INTERNAL
SUPPLY
VCC
THREE-STATE
INPUT
BUFFER
IN
4
SHOOTTHROUGH
PROTECTION
7k
7
3
BG
5
7k
6
GND
13 GND
4447 BD
4447f
6
LTC4447
TIMING DIAGRAM
IN
TG
VIL(TG)
VIL(BG)
VIL(BG)
90%
10%
tr(TG)
tf(TG)
90%
BG
10%
tr(BG)
tpLH(BG)
tpLH(TG)
tf(BG)
tpHL(BG)
4447 TD
tpHL(TG)
OPERATION
Overview
The LTC4447 receives a ground-referenced, low voltage
digital input signal to drive two N-channel power MOSFETs
in a synchronous power supply configuration. The gate
of the low side MOSFET is driven either to VCC or GND,
depending on the state of the input. Similarly, the gate of
the high side MOSFET is driven to either BOOST or TS by
a supply bootstrapped off of the switch node (TS).
VIH(TG)
TG HIGH
TG LOW
TG HIGH
TG LOW
VIL(TG)
IN
VIL(BG)
BG LOW
BG HIGH
BG LOW
BG HIGH
VIH(BG)
4447 F01
Input Stage
The LTC4447 employs a unique three-state input stage with
transition thresholds that are proportional to the VLOGIC
supply. The VLOGIC supply can be tied to the controller
IC’s power supply so that the input thresholds will match
those of the controller’s output signal. Alternatively, VLOGIC
can be tied to VCC to simplify routing. An internal voltage
regulator in the LTC4447 limits the input threshold values
for VLOGIC supply voltages greater than 5V.
The relationship between the transition thresholds and
the three input states of the LTC4447 is illustrated in
Figure 1. When the voltage on IN is greater than the
threshold VIH(TG), TG is pulled up to BOOST, turning the
high side MOSFET on. This MOSFET will stay on until IN
falls below VIL(TG). Similarly, when IN is less than VIH(BG),
BG is pulled up to VCC, turning the low side (synchronous)
MOSFET on. BG will stay high until IN increases above
the threshold VIL(BG).
Figure 1. Three-State Input Operation
The thresholds are positioned to allow for a region in which
both BG and TG are low. An internal resistive divider will
pull IN into this region if the signal driving the IN pin goes
into a high impedance state.
One application of this three-state input is to keep both of
the power MOSFETs off while an undervoltage condition
exists on the controller IC power supply. This can be accomplished by driving the IN pin with a logic buffer that
has an enable pin. With the enable pin of the buffer tied
to the power good pin of the controller IC, the logic buffer output will remain in a high impedance state until the
controller confirms that its supply is not in an undervoltage
state. The three-state input of the LTC4447 will therefore
pull IN into the region where TG and BG are low until the
controller has enough voltage to operate predictably.
4447f
7
LTC4447
OPERATION
The hysteresis between the corresponding VIH and VIL
voltage levels eliminates false triggering due to noise
during switch transitions; however, care should be taken
to keep noise from coupling into the IN pin, particularly
in high frequency, high voltage applications.
VIN
LTC4447
Q1
BOOST
CGD
P1
TG
CGS
N1
HIGH SIDE
POWER
MOSFET
TS
Undervoltage Lockout
The LTC4447 contains undervoltage lockout detectors that
monitor both the VCC and VLOGIC supplies. When VCC falls
below 3.04V or VLOGIC falls below 2.65V, the output pins
BG and TG are pulled to GND and TS, respectively. This
turns off both of the external MOSFETs. When VCC and
VLOGIC have adequate supply voltage for the LTC4447 to
operate reliably, normal operation will resume.
LOAD
INDUCTOR
VCC
Q2
CGD
P2
BG
Q3
CGS
N2
LOW SIDE
POWER
MOSFET
GND
4447 F02
Figure 2. Capacitance Seen by BG and TG During Switching
Adaptive Shoot-Through Protection
Internal adaptive shoot-through protection circuitry
monitors the voltages on the external MOSFETs to ensure
that they do not conduct simultaneously. The LTC4447
does not allow the bottom MOSFET to turn on until the
gate-source voltage on the top MOSFET is sufficiently
low, and vice-versa. This feature improves efficiency by
eliminating cross-conduction current from flowing from
the VIN supply through the MOSFETs to ground during a
switch transition.
Output Stage
A simplified version of the LTC4447’s output stage is
shown in Figure 2. The pull-up device on both the BG and
TG outputs is an NPN bipolar junction transistor (Q1 and
Q2) in parallel with a low resistance P-channel MOSFET
(P1 and P2). This powerful combination rapidly pulls
the BG and TG outputs to their positive rails (VCC and
BOOST, respectively). Both BG and TG have N-channel
MOSFET pull-down devices (N1 and N2) which pull BG
and TG down to their negative rails, GND and TS. An additional NPN bipolar junction transistor (Q3) is present on
BG to increase its pull-down drive current capacity. The
rail-to-rail voltage swing of the BG and TG output pins
is important in driving external power MOSFETs, whose
RDS(ON) is inversely proportional to its gate overdrive
voltage (VGS – V TH).
Rise/Fall Time
Since the power MOSFETs generally account for the majority of power loss in a converter, it is important to quickly
turn them on and off, thereby minimizing the transition
time and power loss. The LTC4447’s peak pull-up current
of 3.2A for both BG and TG produces a rapid turn-on
transition for the MOSFETs. This high current is capable
of driving a 3nF load with an 8ns risetime.
It is also important to turn the power MOSFETs off quickly
to minimize power loss due to transition time; however,
an additional benefit of a strong pull-down on the driver
outputs is the prevention of cross-conduction current. For
example, when BG turns the low side power MOSFET off
and TG turns the high side power MOSFET on, the voltage on the TS pin will rise to VIN very rapidly. This high
frequency positive voltage transient will couple through the
CGD capacitance of the low side power MOSFET to the BG
pin. If the BG pin is not held down sufficiently, the voltage
on the BG pin will rise above the threshold voltage of the
low side power MOSFET, momentarily turning it back on. As
a result, both the high side and low side MOSFETs will be
conducting, which will cause significant cross-conduction
current to flow through the MOSFETs from VIN to ground,
thereby introducing substantial power loss. A similar effect
occurs on TG due to the CGS and CGD capacitances of the
high side MOSFET.
4447f
8
LTC4447
OPERATION
The LTC4447’s powerful parallel combination of the
N-channel MOSFET (N2) and NPN (Q3) on the BG
pull-down generates a phenomenal 4ns fall time on BG
while driving a 3nF load. Similarly, the 0.8Ω pull-down
MOSFET (N1) on TG results in a rapid 7ns fall time with
a 3nF load. These powerful pull-down devices minimize
the power loss associated with MOSFET turn-off time and
cross-conduction current.
APPLICATIONS INFORMATION
Power Dissipation
To ensure proper operation and long-term reliability,
the LTC4447 must not operate beyond its maximum
temperature rating. Package junction temperature can
be calculated by:
TJ = TA + (PD)(θJA)
where:
TJ = junction temperature
TA = ambient temperature
PD = power dissipation
θJA = junction-to-ambient thermal resistance
Power dissipation consists of standby, switching and
capacitive load power losses:
PD = PDC + PAC + PQG
where:
PDC = quiescent power loss
PAC = internal switching loss at input frequency fIN
PQG = loss due turning on and off the external
MOSFET with gate charge QG at frequency fIN
The LTC4447 consumes very little quiescent current. The
DC power loss at VLOGIC = 5V and VCC = 5V is only (730μA
+ 600μA)(5V) = 6.65mW.
At a particular switching frequency, the internal power loss
increases due to both AC currents required to charge and
discharge internal nodal capacitances and cross-conduction currents in the internal logic gates. The sum of the
quiescent current and internal switching current with no
load are shown in the Typical Performance Characteristics
plot of Switching Supply Current vs Input Frequency.
The gate charge losses are primarily due to the large AC
currents required to charge and discharge the capacitance
of the external MOSFETs during switching. For identical
pure capacitive loads CLOAD on TG and BG at switching
frequency fin, the load losses would be:
PCLOAD = (CLOAD)(fIN)[(VBOOST – TS)2 + (VCC)2]
In a typical synchronous buck configuration, VBOOST – TS
is equal to VCC – VD, where VD is the forward voltage drop
of the internal Schottky diode between VCC and BOOST.
If this drop is small relative to VCC, the load losses can
be approximated as:
PCLOAD ≈ 2(CLOAD)(fIN)(VCC)2
Unlike a pure capacitive load, a power MOSFET’s gate
capacitance seen by the driver output varies with its VGS
voltage level during switching. A MOSFET’s capacitive load
power dissipation can be calculated using its gate charge,
QG. The QG value corresponding to the MOSFET’s VGS
value (VCC in this case) can be readily obtained from the
manufacturer’s QG vs VGS curves. For identical MOSFETs
on TG and BG:
PQG ≈ 2(VCC)(QG)(fIN)
To avoid damaging junction temperatures due to power
dissipation, the LTC4447 includes a temperature monitor
that will pull BG and TG low if the junction temperature
exceeds 160°C. Normal operation will resume when the
junction temperature cools to less than 135°C.
4447f
9
LTC4447
APPLICATIONS INFORMATION
Bypassing and Grounding
The LTC4447 requires proper bypassing on the VLOGIC, VCC
and VBOOST – TS supplies due to its high speed switching
(nanoseconds) and large AC currents (amperes). Careless
component placement and PCB trace routing may cause
excessive ringing and under/overshoot.
To obtain the optimum performance from the LTC4447:
• Mount the bypass capacitors as close as possible
between the VLOGIC and GND pins, the VCC and GND
pins, and the BOOST and TS pins. The leads should
be shortened as much as possible to reduce lead
inductance.
• Use a low inductance, low impedance ground plane
to reduce any ground drop and stray capacitance.
Remember that the LTC4447 switches greater than
5A peak currents and any significant ground drop will
degrade signal integrity.
• Plan the power/ground routing carefully. Know where
the large load switching current is coming from and
going to. Maintain separate ground return paths for the
input pin and the output power stage.
• Keep the copper trace between the driver output pin
and the load short and wide.
• Be sure to solder the Exposed Pad on the back side of
the LTC4447 packages to the board. Correctly soldered
to a double-sided copper board, the LTC4447 has a
thermal resistance of approximately 43°C/W. Failure
to make good thermal contact between the exposed
back side and the copper board will result in thermal
resistances far greater.
TYPICAL APPLICATION
LTC7510/LTC4447 12V to 1.5V/30A Digital Step-Down DC/DC Converter with PMBus Serial Interface
12V
5V
R1
SDATA
PMBus
INTERFACE
SCLK
VD33
SMB_AL_N
LTC7510
POWER
MANAGEMENT
INTERFACE
C5
0.22μF
V12SEN
VCC
VD25
C2
OUTEN
BOOST
R2
+
PWRGD
C4
C1 C3
GND
PWM
MULTIPHASE
INTERFACE
TG
VLOGIC
LTC4447
VCC
TS
IN
1μF
SYNC_IN
RCM
SYNC_OUT
BG
GND
M1
RJK0305
×2
M2
RJK0301
×2
L1
0.3μH
R3
C6
VOUT
+
330μF
×6
D1
TEMPSEN
LOAD
FAULT
OUTPUTS
FAULT1
FAULT2
ISENN
RSENSE
ISENP
VSENP
VSENN
I-SHARE
IOUT/ISH
ISH_GND
1k
SADDR
1k
VSET
RTN
4447 TA02
1k
FSET
1k
RESET_N
VTRIM
1k
IMAXSET
4447f
10
LTC4447
PACKAGE DESCRIPTION
DDMA Package
12-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1743 Rev A)
0.70 ±0.05
1.19 ±0.05
0.93 ±0.05
2.25 REF
0.57 ±0.05
2.38 ±0.05
1.35 ±0.05
3.50 ±0.05
0.81 ±0.05
2.10 ±0.05
PACKAGE
OUTLINE
1.07 ±0.05
0.25 ± 0.05
0.45 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.115
TYP
7
3.00 ±0.10
0.11 ± 0.05
12
0.40 ± 0.10
2.38 ±0.10
3.00 ±0.10
0.81 ± 0.10
PIN 1
TOP MARK
(SEE NOTE 6)
0.200 REF
R = 0.05
TYP
0.63 ± 0.05
1.35 ± 0.10
6
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
1
0.23 ± 0.05
0.45 BSC
0.75 ±0.05
2.25 REF
(DD12MA) DFN 0507 REV A
BOTTOM VIEW—EXPOSED PAD
0.00 – 0.05
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD AND TIE BARS SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
4447f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
LTC4447
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC1154
High Side Micropower MOSFET Driver
Internal Charge Pump 4.5V to 18V Supply Range
LTC1155
Dual Micropower High/Low Side Driver
Internal Charge Pump 4.5V to 18V Supply Range
Quad Protected High Side MOSFET Driver
8V to 48V Supply Range, tON = 200μs, tOFF = 28μs
®
LT 1161
LTC1163
Triple 1.8V to 6V High Side MOSFET Driver
1.8V to 6V Supply Range, tON = 95μs, tOFF = 45μs
LTC1693 Family
High Speed Single/Dual N-Channel MOSFET Drivers
1.5A Peak Output Current, 4.5V ≤ VIN ≤ 13.2V
LTC3900
Synchronous Rectifier Driver for Forward Converters
Pulse Drive Transformer Synchronous Input
LTC3901
Secondary Side Synchronous Driver for Push-Pull and
Full-Bridge Converters
Gate Drive Transformer Synchronous Input
LTC4440
High Speed, High Voltage High Side Gate Driver
High Side Source Up to 100V, 8V ≤ VCC ≤ 15V
LTC4440-5
High Speed, High Voltage High Side Gate Driver
High Side Source Up to 80V, 4V ≤ VCC ≤ 15V
LTC4441
6A MOSFET Driver
6A Peak Output Current, Adjustable Gate Drive from 5V to 8V,
5V ≤ VIN ≤ 25V
LTC4442/LTC4442-1 High Speed Synchronous N-Channel MOSFET Driver
5A Peak Output Current, Three-State Input, 38V Maximum
Input Supply Voltage, 6V ≤ VCC ≤ 9.5V, MS8E Package
LTC4443/LTC4443-1 High Speed Synchronous N-Channel MOSFET Driver
5A Peak Output Current, Internal Schottky Diode, 38V Maximum
Input Supply Voltage, 6V ≤ VCC ≤ 9.5V, 3mm × 3mm DFN-12
LTC4444/LTC4444-5 High Voltage/High Speed Synchronous N-Channel MOSFET
Driver
3A Peak Output Current, 100V Maximum Input Supply Voltage,
4.5V ≤ VCC ≤ 13.5V, with Adaptive Shoot Through Protection
LTC4446
High Voltage High Side/Low Side N-Channel MOSFET Driver
3A Output Current, 100V Input Supply Voltage, 7.2V ≤ VCC ≤ 13.5V,
without Adaptive Shoot Through Protection
LTC7510
Digital DC/DC Controller with PMBus Interface
Digital Controller, PMBus Serial Interface, 150kHz to 2MHz
Switching Frequency
4447f
12 Linear Technology Corporation
LT 0608 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
工商网监
湘ICP备2023018690号
