SEMTECH SC1205CS

HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
PRELIMINARY - December 7, 1999
SC1205
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
DESCRIPTION
FEATURES
The SC1205 is a cost effective Dual MOSFET Driver
designed for switching High and Low side Power
MOSFETs. Each driver is capable of driving a
3000pF load in 20ns rise/fall time and has a 20ns max
propagation delay from input transition to the gate of
the power FET’s. An internal Overlap Protection Circuit prevents shoot-through from Vin to GND in the
main switching and synchronous MOSFET’s. The
Overlap Protection circuit ensures the Bottom FET
does not turn on until the Top FET source has reached
a voltage low enough to prevent cross-conduction.
•
The high current drive capability (2A peak) allows fast
switching, thus reducing switching losses at high
(1MHz) PWM frequencies. The high voltage CMOS
process allows operation from 5-25 Volts at top MOSFET drain, thus making SC1205 suitable for battery
powered applications. Connecting Enable pin (EN) to
logic low shuts down both drives and reduces operating current to less than 10uA.
An Under-Voltage-Lock-Out circuit is included to guarantee that both driver outputs are low when the 5V
logic level is less than or equal to 4.4V (typ) at supply
ramp up (4.35V at supply ramp down). An Internal
temperature sensor shuts down all drives in the event
of overtemperature. SC1205 is fabricated utilizing
CMOS technology for low quiescent current. The
SC1205 is offered in a standard SO-8 package.
•
•
•
•
•
•
•
•
Fast rise and fall times (15ns typical with 3000pf
load)
2Amp peak drive current
14ns max Propagation delay (BG going low)
Adaptive Non-overlapping Gate Drives provide
shoot-through protection
Floating top drive switches up to 25V
Under-Voltage lock-out
Overtemperature protection
Less than 10uA supply current when EN is low
Low cost
APPLICATIONS
•
•
•
•
•
High Density sunchronous power supplies
Motor Drives/Class-D amps/Half bridge drivers
High frequency (to 1.2 MHz) operation allows use
of small inductors and low cost caps in place of
electrolytics
Portable computers
Battery powered applications
ORDERING INFORMATION
(1)
DEVICE
SC1205CS
PACKAGE
TEMP. RANGE (T J)
SO-8
0 - 125°C
Note:
(1) Add suffix ‘TR’ for tape and reel.
PIN CONFIGURATION
BLOCK DIAGRAM
Top View
(SO-8)
1
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
ABSOLUTE MAXIMUM RATINGS
Parameter
Symbol
VCC Supply Voltage
BST to PGND
BST to DRN
DRN to PGND
OVP_S to PGND
Input pin
Continuous Power Dissipation
Conditions
Maximum
Units
Tamb = 25°C, TJ = 125°C
Tcase = 25°C, TJ = 125°C
7
30
7
25
10
-0.3 to 7.3
0.66
2.56
V
V
V
V
V
V
W
θϑΧ
40
°C/W
θJA
150
°C/W
TJ
0 to +125
°C
TSTG
-65 to +150
°C
TLEAD
300
°C
VMAX5V
VMAXBST-PGND
VMAXBST-DRN
VMAXDRN-PGN
VMAXOVP_S-PGND
CO
Pd
Thermal Resistance Junction to Case
Thermal Resistance Junction to
Ambient
Operating Temperature Range
Storage Temperature Range
Lead Temperature (Soldering) 10 sec
NOTE:
(1) Specification refers to application circuit in Figure 1.
ELECTRICAL CHARACTERISTICS (DC OPERATING SPECIFICATIONS)
Unless specified: -0 < θJ < 125°C; VCC = 5V; 4V < VBST < 26V
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
VCC
VCC
4.15
5
6.0
Quiescent Current, operating
Iq_op
VCC = 5V, CO = 0V
Quiescent Current
Iq_stby
UNITS
POWER SUPPLY
Supply Voltage
1
EN = 0V
V
ma
10
µA
4.6
V
UNDER-VOLTAGE LOCKOUT
Start Threshold
Hysteresis
Logic Active Threshold
VSTART
VhysUVLO
VACT
4.2
4.4
0.05
V
1.5
V
2
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
ELECTRICAL CHARACTERISTICS (DC OPERATING SPECIFICATIONS) Cont.
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX UNITS
CO
High Level Input Voltage
VIH
Low Level Input Voltage
VIL
2.0
V
0.8
V
THERMAL SHUTDOWN
Over Temperature Trip Point
TOTP
165
°C
Hysteresis
THYST
10
°C
IPKH
2
A
1
Ω
HIGH-SIDE DRIVER
Peak Output Current
Output Resistance
RsrcTG
RsinkTG
duty cycle < 2%, tpw < 100µs,
TJ = 125°C, VBST - VDRN = 4.5V,
VTG = 4.0V (src)+VDRN
or VTG = 0.5V (sink)+VDRN
.7
Ω
LOW-SIDE DRIVER
Peak Output Current
Output Resistance
IPKL
RsrcBG
RsinkBG
duty cycle < 2%, tpw < 100µs,
TJ = 125°C
VV_5 = 4.6V, VBG = 4V (src),
or VLOWDR = 0.5V (sink)
2
A
1.2
Ω
1.0
Ω
3
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
AC OPERATING SPECIFICATIONS
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX UNITS
rise time
trTG,
CI = 3nF, VBST - VDRN = 4.6V,
T J = 125°C
14
23
ns
fall time
tf TG
CI = 3nF, VBST - VDRN = 4.6V,
T J = 125°C
12
19
ns
propagation delay time,
TG going high
tpdhTG
CI = 3nF, VBST - VDRN = 4.6V,
T J = 125°C
20
32
ns
propagation delay time,
TG going low
tpdlTG
CI = 3nF, VBST - VDRN = 4.6V,
T J = 125°C
15
24
ns
rise time
trBG
CI = 3nF, VV_5 = 4.6V,
T J = 125°C
15
24
ns
fall time
trBG
CI = 3nF, VV_5 = 4.6V,
T J = 125°C
13
21
ns
propagation delay time
BG going high
tpdhBGHI
CI = 3nF, VV_5 = 4.6V,
T J = 125°C, DRN < 1V
12
19
ns
progagation delay time
BG going low
tpdlBG
CI = 3nF, VV_5 = 4.6V,
T J = 125°C
7
12
ns
V_5 ramping up
tpdhUVLO
EN is High
10
us
V_5 ramping down
tpdlUVLO
EN is High
10
us
HIGH-SIDE DRIVER
LOW-SIDE DRIVER
UNDER-VOLTAGE LOCKOUT
4
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
PIN DESCRIPTION
Pin #
Pin Name
Pin Function
1
DRN
This pin connects to the junction of the switching and synchronous
MOSFET’s. This pin can be subjected to a -2V minimum relative to
PGND without affecting operation.
2
TG
Output gate drive for the switching (high-side) MOSFET.
3
BST
Bootstrap pin. A capacitor is connected between BST and DRN pins to
develop the floating bootstrap voltage for the high-side MOSFET. The
capacitor value is typically between 0.1µF and 1µF (ceramic).
4
CO
TTL-level input signal to the MOSFET drivers.
5
EN
When high, this pin enables the internal circuitry of the device. When
low, TG, BG and PRDY are forced low and the supply current (5V) is
less than 10µA.
6
VS
+5V supply. A .22-1µF ceramic capacitor should be connected from 5V
to PGND very close to this pin.
7
BG
Output drive for the synchronous MOSFET.
8
PGND
Ground.
NOTE:
(1) All logic level inputs and outputs are open collector TTL compatible.
TIMING DIAGRAM
5
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
Figure 1 - Timing characteristics while driving a 3nf load at Tamb = 125°C
after CO low to high transition.
Ch1. CO input
going high (start
of cycle)
Ch2. BG drive
Ch3. TG drive
Cload = 3 nf
C_delay = 0
Ch1. DRN (phase
node) voltage going low
Ch2. BG going
high
Cload = 3 nf
C_delay = 0
Figure 2-Timing characteristics while driving a 3nf load at Tamb = 125°C
after DRN voltage transition to a low voltage (DRN <1V)
6
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
7
B121
A121
VCC_L2
VID[4]
9
10
11
12
13
14
15
16
INPUT
+5V
8
7
6
5
4
3
2
1
Vout/Clk switch
S1
1u,16V
1u,16V
1u,16V
C8
1000uf,16V
.01
C26
3.9k
R6
EN
C11
.1
C9
100
R7
C12
.1
C27
10k
35k
R10
10
9
8
7
6
5
4
3
2
vcc5v 1
R38
10
R1
OC-
OC+
Vid4
Vid3
Vid2
Vid1
Vid0
Rref
NC
U1
Comp
FB
Bgout
FBG
Enable
GND
Drv0
Drv1
Outv
Vcc12v
SC1142CSW
35k
R10
Vcc5v
C23
.022
11
12
13
14
C25
R8
.022
6k
VOUT
10
R43
10
R44
300k
R11
15 EN
16
17
18
19
20
VIN
SC1205
CO
EN
VS
EN
4
5
6
U12
3
BG
TG
7
1
2
7
1
2
0
R13
*
51
*
OPTIONAL
R12
*
FDB7030
Q4
IR7811
Q3
FDB7030
51
4.7
R45
0 R41
VIN
0
R40
Q1
IR7811
4.7
R45
Q2
VIN
0 R39
R42
C31
C4
22u,10V 1u,10V
BG
DRN
BST
TG
DRN
SC1205
CO
EN
VS
D11
4
5
6
5819
D10
GND
C16
22u,10V
.1 5819
C57
U11
1u,10V
C28
R9
10
C55
.1
C15
1u,10V
8
Long PCB Trace
3
BST
1000uf
C10
GND
C5 C6 C7
100UF
10uf
8
+12V
R14
*
51
VOUT
R15
*
51
D2
SS12
.6uh
L2
.1
C58
D1
SS12
.6uh
L1
C56
.1
22u,10V
C36
22u,10V
C32
22u,10V
C29
22u,10V
C24
22u,10V
C22
22u,10V
C21
22u,10V
C20
22u,10V
C19
22u,10V
C18
22u,10V
C14
22u,10V
C13
1500uf
C17
1500uf
C30
Date:
B
Size
Title
Tuesday, June 29, 1999
SC1142EVB-A
SC1142-1205 Evaluation Board
PentiumII
Document Number
A120
A119
A118
A117
A116
A115
A114
A113
A112
A111
A110
A109
A108
A107
A106
A105
A104
A103
A102
A101
A100
A99
A98
A97
A96
A95
A94
A93
A92
A91
A90
A89
A88
A87
A86
A85
A84
A83
A82
A81
A80
A79
A78
A77
A76
A75
A74
A73
A72
A71
A70
A69
A68
A67
A66
A65
A64
A63
A62
A61
A60
A59
A58
A57
A56
A55
A54
A53
A52
A51
A50
A49
A48
A47
A46
A45
A44
A43
A42
A41
A40
A39
A38
A37
A36
A35
A34
A33
A32
A31
A30
A29
A28
A27
A26
A25
A24
A23
A22
A21
A20
A19
A18
A17
A16
A15
A14
A13
A12
A11
A10
A9
A8
A7
A6
A5
A4
A3
A2
A1
1
2
3
4
5
6
VID[1]
VID[2]
GND
$PIN116
$PIN115
$PIN114
GND
$PIN112
$PIN111
$PIN110
GND
$PIN108
$PIN107
$PIN106
GND
$PIN104
$PIN103
$PIN102
GND
$PIN100
$PIN99
$PIN98
GND
$PIN96
$PIN95
$PIN94
GND
$PIN92
$PIN91
$PIN90
GND
$PIN88
$PIN87
$PIN86
GND
$PIN84
$PIN83
$PIN82
GND
$PIN80
$PIN79
$PIN78
GND
$PIN76
$PIN75
$PIN74
GND
$PIN72
$PIN71
$PIN70
GND
$PIN68
$PIN67
$PIN66
GND
$PIN64
$PIN63
$PIN62
GND
$PIN60
$PIN59
$PIN58
GND
$PIN56
$PIN55
$PIN54
GND
$PIN52
$PIN51
$PIN50
GND
$PIN48
$PIN47
$PIN46
GND
$PIN44
$PIN43
$PIN42
GND
$PIN40
$PIN39
$PIN38
GND
$PIN36
$PIN35
$PIN34
GND
$PIN32
$PIN31
$PIN30
GND
$PIN28
$PIN27
$PIN26
GND
$PIN24
$PIN23
$PIN22
GND
$PIN20
$PIN19
$PIN18
GND
$PIN16
Reserved
THERMTRIP#
GND
TESTHI
PWRGOOD
TDO
GND
TDI
IGNNE#
FERR#
GND
A20M#
IERR#
VCC_VTT
GND
VCC_VTT
J1
VID[0]
VID[3]
$PIN238
VCC_L2
$PIN236
$PIN235
$PIN234
VCC_L2
$PIN232
$PIN231
$PIN230
VCC5
$PIN228
$PIN227
$PIN226
VCC_CORE
$PIN224
$PIN223
$PIN222
$PIN221
$PIN220
$PIN219
$PIN218
VCC_CORE
$PIN216
$PIN215
$PIN214
VCC_CORE
$PIN212
$PIN211
$PIN210
VCC_CORE
$PIN208
$PIN207
$PIN206
VCC_CORE
$PIN204
$PIN203
$PIN202
$PIN201
$PIN200
$PIN199
$PIN198
VCC_CORE
$PIN196
$PIN195
$PIN194
VCC_CORE
$PIN192
$PIN191
$PIN190
VCC_CORE
$PIN188
$PIN187
$PIN186
VCC_CORE
$PIN184
$PIN183
$PIN182
$PIN181
$PIN180
$PIN179
$PIN178
VCC_CORE
$PIN176
$PIN175
$PIN174
VCC_CORE
$PIN172
$PIN171
$PIN170
VCC_CORE
$PIN168
$PIN167
$PIN166
VCC_CORE
$PIN164
$PIN163
$PIN162
$PIN161
$PIN160
$PIN159
$PIN158
VCC_CORE
$PIN156
$PIN155
$PIN154
VCC_CORE
$PIN152
$PIN151
$PIN150
VCC_CORE
$PIN148
$PIN147
$PIN146
VCC_CORE
$PIN144
$PIN143
$PIN142
$PIN141
$PIN140
$PIN139
$PIN138
VCC_CORE
$PIN136
$PIN135
$PIN134
VCC_CORE
$PIN132
$PIN131
$PIN130
VCC_VTT
$PIN128
$PIN127
$PIN126
VCC_VTT
$PIN124
$PIN123
$PIN122
$PIN121
+5V
B120
B119
B118
B117
B116
B115
B114
B113
B112
B111
B110
B109
B108
B107
B106
B105
B104
B103
B102
B101
B100
B99
B98
B97
B96
B95
B94
B93
B92
B91
B90
B89
B88
B87
B86
B85
B84
B83
B82
B81
B80
B79
B78
B77
B76
B75
B74
B73
B72
B71
B70
B69
B68
B67
B66
B65
B64
B63
B62
B61
B60
B59
B58
B57
B56
B55
B54
B53
B52
B51
B50
B49
B48
B47
B46
B45
B44
B43
B42
B41
B40
B39
B38
B37
B36
B35
B34
B33
B32
B31
B30
B29
B28
B27
B26
B25
B24
B23
B22
B21
B20
B19
B18
B17
B16
B15
B14
B13
B12
B11
B10
B9
B8
B7
B6
B5
B4
B3
B2
B1
Sheet
1
of
1
Figure 3-SC1205 Evaluation
Board
Rev
N/C
PRELIMINARY - December 7, 1999
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
BILL OF MATERIAL
Item Qty
Reference
Value
Manufacturer
1
3
C4,C15,C28
1u,10V, Cer.
AVX, Murata
2
3
C5,C6,C7
1u,16V, Cer.
AVX, Murata
3
1
C8
1000uF, 16V
Nichicon, any
4
1
C9
100uF
Nichicon, any
5
1
C10
1000uf
Nichicon, any
7
1
C12
10uF
Nichicon, any
8
13
C13,C14,C16,C18,C19,C20,C21,C22,C24,C29,C31,C32,
C36
9
2
C17,C30
1500uf
10
2
C23,C25
.022
Avx, any
11
1
C26
.01
Avx, any
12
6
C11,C27,C55,C56,C57,C58
.1
Avx, any
14
2
D1,D2
SS12
General Instruments, any
15
2
D10,D11
5819
General Instruments, any
16
1
J1
Input
17
2
L1,L2
.6uh
18
2
Q1,Q3
IR7811
Int. Rectifier
(310) 252-7099
19
2
Q2,Q4
FDB7030
Fairchild Semi.
(408) 822-2000
20
4
R1,R9,R43,R44
21
1
22
22u, 10V
Murata
(GRM235Y5V226Z010)
Nichicon, Sanyo
13
Falco, P/N: TO2508 or
SDIP0804-608M
(305) 662-9076
10
any
R6
3.9k
any
1
R7
100
any
23
1
R8
6k
any
24
1
R10
35k
any
25
1
R11
300k
any
26
4
R12,R13,R14,R15
27
1
R38
28
4
R39,R40,R41,R42
29
1
R45
30
1
S1
Vout/Clk switch
31
1
U2
Pentium II™
Slot 1 Connector
32
1
U1
SC1142CSW
Semtech, (805) 499-2111
33
2
U11,U12
SC1205S
Semtech, (805) 499-2111
51
any, Required in asynch.
operation
10K
any
0
any
4.7
any
Digikey
8
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
APPLICATION INFORMATION:
SC1205 is a high speed, smart dual MOSFET driver.
It is designed to drive Low Rds_On power MOSFET’s
with ultra-low rise/fall times and propagation delays.
As the switching frequencies of PWM controllers is
increased to reduce power supply and Class-D amplifier volume and cost, fast rise and fall times are necessary to minimize switching losses (TOP MOSFET)
and reduce Dead-time (BOTTOM MOSFET). While
2
Low Rds_On MOSFET’s present a power saving in I R
losses, the MOSFET’s die area is larger and thus the
effective input capacitance of the MOSFET is increased. Often a 50% decrease in Rds_On more than
doubles the effective input gate charge, which must be
supplied by the driver. The Rds_On power savings
can be offset by the switching and dead-time losses
with a sub-optimum driver. While discrete solution
can achieve reasonable drive capability, implementing
shoot-through, programmable delay and other housekeeping functions necessary for safe operation can
become cumbersome and costly. The SC120X family
of parts presents a total solution for the high-speed,
high power density applications. Wide input supply
range of 4.5V-25V allows use in battery powered applications, new high voltage, distributed power servers
as well as Class-D amplifiers.
THEORY OF OPERATION
The control input (CO) to the SC1205 is typically supplied by a PWM controller that regulates the power
supply output. (See Application Evaluation
Schematic, Figure 3). The timing diagram demonstrates the sequence of events by which the top and
bottom drive signals are applied. The shoot-through
protection is implemented by holding the bottom FET
off until the voltage at the phase node (intersection of
top FET source, the output inductor and the bottom
FET drain) has dropped below 1V. This assures that
the top FET has turned off and that a direct current
path does not exist between the input supply and
ground, a condition which both the top and bottom
FET’s are on momentarily. The top FET is also prevented from turning on until the bottom FET is off.
This time is internally set to 20ns.
to figure 3) shows a two-phase synchronous design
with all surface mountable components.
While components connecting to EN are relatively
non-critical, tight placement and short,wide traces
must be used in layout of The Drives, DRN, and especially PGND pin. The top gate driver supply voltage is
provided by bootstrapping the +5V supply and adding
it the phase node voltage (DRN). Since the bootstrap
capacitor supplies the charge to the top gate, it must
be less than .5” away from the SC1205. Ceramic X7R
capacitors are a good choice for supply bypassing
near the chip. The Vcc pin capacitor must also be less
than .5” away from the SC1205. The ground node of
this capacitor, the SC1205 PGND pin and the Source
of the bottom FET must be very close to each other,
preferably with common PCB copper land with multiple vias to the ground plane (if used). The parallel
Shottkey must be physically next to the Bottom FETS
Drain and source. Any trace or lead inductance in
these connections will drive current way from the
Shottkey and allow it to flow through the FET’s Body
diode, thus reducing efficiency.
PREVENTING INADVERTENT BOTTOM FET
TURN-ON
At high input voltages, (12V and greater) a fast turn-on
of the top FET creates a positive going spike on the
Bottom FET’s gate through the Miller capacitance,
crss of the bottom FET. The voltage appearing on the
gate due to this spike is:
Vspike=Vin*crss/(Crass+ciss)
Where Ciss is the input gate capacitance of the bottom FET. This is assuming that the impedance of the
drive path is too high compared to the instantaneous
impedance of the capacitors. (since dV/dT and thus
the effective frequency is very high). If the BG pin of
the SC1205 is very close to the bottom FET, Vspike
will be reduced depending on trace inductance, rate of
rise of current,etc.
LAYOUT GUIDELINES
As with any high speed , high current circuit, proper
layout is critical in achieving optimum performance of
the SC1205. The Evaluation board schematic (Refer
9
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
While not shown in Figure 3, a capacitor may be
added from the gate of the Bottom FET to its source,
preferably less than .1” away. This capacitor will be
added to Ciss in the above equation to reduce the effective spike voltage, Vspike.
The selection of the bottom MOSFET must be done
with attention paid to the Crss/Ciss ratio. A low ratio
reduces the Miller feedback and thus reduces Vspike.
Also MOSFETs with higher Turn-on threshold voltages
will conduct at a higher voltage and will not turn on
during the spike. The MOSFET shown in the
schematic (figure 3) has a 2 volt threshold and will require approximately 4.5 volts Vgs to be conducting,
thus reducing the possibility of shoot-through. A zero
ohm bottom FET gate resistor will obviously help
keeping the gate voltage low.
Ultimately, slowing down the top FET by adding gate
resistance will reduce di/dt which will in turn make the
effective impedance of the capacitors higher, thus allowing the BG driver to hold the bottom gate voltage
low. It does this at the expense of increased switching
times ( and switching losses) for the top FET.
can often eliminate the EMI issue. If double pulsing is
caused due to excessive ringing, placing 4.7-10 ohm
resistor between the phase node and the DRN pin of
the SC1205 should eliminate the double pulsing.
Proper layout will guarantee minimum ringing and
eliminate the need for external components. Use of
SO-8 or other surface mount MOSFETs while increasing thermal resistance, will reduce lead inductance as
well as radiated EMI.
OVER TEMP SHUTDOWN
The SC1205 will shutdown by pulling both driver if its
junction temperature, Tj, exceeds 165 °C.
RINGING ON THE PHASE NODE
The top MOSFET source must be close to the bottom
MOSFET drain to prevent ringing and the possibility of
the phase node going negative. This frequency is
determined by:
Fring =1/(2¶* Sqrt(Lst*Coss))
Where:
Lst = The effective stray inductance of the top FET
added to trace inductance of the connection between
top FET’s source and the bottom FET’s drain added to
the trace resistance of the bottom FET’s ground connection.
Coss=Drain to source capacitance of bottom FET. If
there is a Shottkey used, the capacitance of the Shottkey is added to the value.
Although this ringing does not pose any power losses
due to a fairly high Q, it could cause the phase node
to go too far negative, thus causing improper operation, double pulsing or at worst driver damage. On the
SC1205, the drain node, DRN, can go as far as 2V
below ground without affecting operation or sustaining
damage.
The ringing is also an EMI nuisance due to its high
resonant frequency. Adding a capacitor, typically
1000-2000pf, in parallel with Coss of the bottom FET
10
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
Figure 4-Timing diagram:
Ch1:CO input
Ch2:TG drive
Ch3:BG non-overlap drive
Ch4:phase node
Iout=20A (10A/phase)
Refer to Eval. Schematic
(fig.3)
Figure 5-Timing diagram:
Rise/Fall times
Ch1:TG drive
Ch2:BG drive
Cursor:TpdhTG
Iout=20A (10A/phase)
Refer to Eval. Schematic
(fig.3)
11
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
HIGH SPEED SYNCHRONOUS POWER
MOSFET DRIVER
SC1205
PRELIMINARY - December 7, 1999
OUTLINE DRAWING - SO-8
JEDEC
REF: MS-012AA
LAND PATTERN - SO-8
ECN99-742
12
© 1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320