LT1681
Dual Transistor
Synchronous Forward Controller
DESCRIPTIO
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FEATURES
High Voltage: Operation Up to 72V
Synchronizable Operating Frequency and Output
Switch Phase for Multiple Controller Systems
Fixed Frequency Operation to 350kHz
Adaptive and Adjustable Blanking
Synchronous Rectifier Driver
Local 1% Voltage Reference
Undervoltage Lockout Protection with Hysteresis
Input Overvoltage Protection
Programmable Start Inhibit
Transformer Primary Saturation Protection
Optocoupler Feedback Support
Soft-Start Control
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The LT ®1681 controller simplifies the design of high power
synchronous dual transistor forward DC/DC converters. The
part employs fixed frequency current mode control and
supports both isolated and nonisolated topologies. The IC
drives external N-channel power MOSFETs and operates with
input voltages up to 72V.
The LT1681’s operating frequency is programmable and can
be synchronized up to 350kHz. Switch phase is also controlled during synchronized operation to accommodate multiple converter systems. Internal logic guarantees 50% maximum duty cycle operation to prevent transformer saturation.
The LT1681 incorporates a soft-start feature that provides a
controlled increase in supplied current during start-up and
after an undervoltage lockout or overvoltage/overcurrent
event.
The part is available in a 20-lead wide SO package to support
high voltage pin-to-pin clearance.
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APPLICATIO S
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Isolated Telecommunication Systems
Personal Computers and Peripherals
Lead Acid Battery Backup Systems
Automotive and Heavy Equipment
, LTC and LT are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)
L2
4.1µH
L1
4.7µH
VOUT = 5V
IOUT = 7A
VOUT+
•
VIN+
6
Q1
C4
1.5µF
100V
5
MURS120T3
C3
1.5µF
100V
•
•
MURS120T3
C2
22µF
100V
8
4
Q3
0.025Ω
1/2W
•
+
7
1
2
3
10Ω
0.25W
MBR0540T1
1nF
100V
10 1nF
11 100V 10Ω
0.25W
12
+
Q6
9
4.7Ω
T1 Q5
VIN–
C2:SANYO 100MV22AX
C3, C4: VITRAMON VJ1825Y155MXB
C5: 4X KEMET T510X337KO10AS
L1: COILCRAFT DO1608C-472
L2: PANASONIC ETQP6F4R1LF4
Q1,Q3:100V SILICONIX SUD40N10-25
Q5,Q6: SILICONIX Si4450
T1:COILTRONICS VP5-1200
Q10: ON SEMI MMBT3906LTI
73.2k
1%
270k
0.25W
20k
VOUT–
ZVN3310F
1OV
BIAS
CMPZ5248B
18V
0.1µF
68µF
20V
10k
+
1nF 24k
BAT54
10k
MMBD914LT1
0.1µF
100V
20
17
19
18
16
11
12
15
VCC VBST BLKSENS TG BSTREF BG SENSE TMAX PGND 13
SG
LT1681
OVLO
9
1
THERM SYNC SGND SS VC VFB
SHDN 5VREF FSET
1.24k
1%
6
5
52.3k
Q10
1µF
3
7
4
8
10
100Ω
150pF
4.7nF
0.01µF
100Ω
FZT690
4.7µF
16V
5V
OUT
2
56k
1OV
BIAS
330pF
BAS21
14
CMPZ5248B
15V
C5
330µF
10V
3.3Ω
0.047µF
3.01k
1%
LTC1693-2
6
VCC1
VCC2
5
IN2
OUT2
7
1
IN1
OUT1
2
4
GND2
GND1
2k
0.22µF
50V
CMPZ5242B
12V
8
3
51Ω
1681 TA01
1k
1%
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LT1681
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
Supply Voltages
Power Supply (VCC) ............................. – 0.3V to 20V
Topside Supply (VBST) ................... VBSTREF – 0.3V to
VBSTREF + 20V (VBST(MAX) = 90V)
Topside Reference Pin (VBSTREF) .......... – 0.6V to 75V
Input Voltages
SHDN Pin .................................. – 0.3V to VCC + 0.3V
All Other Inputs ..................... – 0.3V to 5VREF + 0.3V
Maximum Currents
5VREF Pin ........................................ – 85mA to 10mA
FSET Pin ............................................. – 2mA to 5mA
All Other Inputs .................................. – 2mA to 2mA
Operating Ambient Temperature Range
LT1681E (Note 4) .............................. – 40°C to 85°C
LT1681I ............................................. – 40°C to 85°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
TOP VIEW
SHDN 1
20 VBST
OVLO 2
19 TG
THERM 3
LT1681ESW
LT1681ISW
18 BSTREF
SGND 4
17 BLKSENS
5VREF 5
16 BG
FSET 6
15 PWRGND
SYNC 7
14 VCC
SS 8
13 SG
VFB 9
12 IMAX
VC 10
11 SENSE
SW PACKAGE
20-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 85°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
9
12
18
V
17
22
25
mA
mA
800
1200
µA
Supply and Protection
VCC
Operating Supply Voltage Range
ICC
DC Active Supply Current
●
(Note 2)
●
IBST
VSHDN
DC Active UVL Supply Current
VSHDN > 1.35V, VCC = 8V
DC Standby Supply Current
VSHDN < 0.3V
DC Active Supply Current
TG Logic High (Note 2)
DC Standby Supply Current
VSHDN < 0.3V
Shutdown Rising Threshold
Shutdown Threshold Hysteresis
ISS
Soft-Start Charge Current
VSS
Soft-Start Reset Threshold
VCCUVLO
Undervoltage Lockout Threshold
VSS = 2V
Boost UVLO Hysteresis
µA
0.5
5
●
8.5
●
1.15
●
●
Falling Edge
Rising Edge
Falling Edge
Rising Edge
mA
µA
0.1
1.25
1.35
V
100
150
200
mV
–14
– 10
–6
µA
225
Undervoltage Lockout Hysteresis
VBSTUVLO Boost Undervoltage Lockout
(VBST-BSTREF)
●
●
●
8.0
8.3
8.40
8.75
●
0.25
0.35
●
●
5.7
6.5
6.4
7.0
●
0.3
0.6
mV
8.60
8.95
V
V
V
7.1
7.5
V
V
V
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LT1681
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
4.85
4.80
5
●
5.10
5.15
V
V
●
20
45
mA
1
Ω
5V External Reference
V5VREF
5V Reference Voltage
0 ≤ (I5VREF – IVC) < 20mA
I5VREFSC
Short-Circuit Current
Source, IVC = 0
R5VREF
Output Impedance
0 ≤ (I5VREF – IVC) < 20mA
Error Amplifier Reference Voltage
Measured at Feedback Pin
Error Amp
VFB
●
IFB
Feedback Input Current
AV
Error Amplifier Voltage Gain
IVC
Error Amplifier Current Limit
VVC
GBW
1.242
1.225
VFB = VREF
1.250
1.258
1.265
–50
V
V
nA
72
dB
25
1
mA
mA
Zero Current Output Voltage
1.4
V
Maximum Output Voltage
3.2
V
Gain Bandwidth Product
Source
Sink
●
●
10
0.5
(Note 3)
1
MHz
12
V/V
Current Sense and Blanking
AV
Amplifier DC Gain
ISENSE
Input Bias Current
VSENSE
Current Limit Threshold
tD
●
135
130
●
4.5
Current Sense to Switch Delay
Blanking Input Bias Current
tMIN
Switch Minimum On Time
150
165
170
175
VBLKSENS Blanking Input Threshold
IBLKSENS
µA
– 275
Measured at SENSE Pin
VBLKSENS = VBG, Measured at BG Output
5
mV
mV
ns
5.5
V
–2
µA
250
ns
IMAX Sense
IIMAX
Input Bias Current
VIMAX
IMAX Threshold (Rising Edge)
IMAX Threshold Hysteresis
Measured at IMAX Input
Measured at IMAX Input
tP
IMAX Output Switch Disable Delay
Measured at BG Output
µA
– 250
●
320
360
140
400
130
mV
mV
ns
THERM and OVLO Fault Detectors
VTHERM/
VOVLO
Threshold (Rising Edge)
Threshold Hysteresis
tP
Fault Delay to Output Disable
●
●
1.2
20
50mV Overdrive
1.25
40
1.3
60
650
V
mV
ns
Oscillator and Synchronization Decoder
fOSC
Oscillator Frequency, Free Run
Measured at FSET Pin
Frequency Programming Error, Free Run
fOSC ≤ 500kHz (Note 3)
IFSET
FSET Input Bias Current
FSET Charging, VFSET = 2V
VSYNC
SYNC Logic High Input Threshold
SYNC Logic Low Input Threshold
Positive-Going Edge
Negative-Going Edge
fSYNC
SYNC Frequency
tH, L
Maximum SYNC Pulse Width
(Logic High or Logic Low)
fOSC = Oscillator Free-Run Frequency
700
●
–10
5
50
●
●
0.8
●
fOSC/2
1.4
1.4
kHz
%
nA
2
350
1/fOSC
V
V
kHz
s
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LT1681
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VTS = 0V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
11
11.5
0.1
0.5
V
V
Output Drivers
VTG
TG On Voltage
TG Off Voltage
tTGr/f
TG Rise/Fall Times
VBG
BG On Voltage
BG Off Voltage
tBGr/f
BG Rise/Fall Times
VSG
SG On Voltage
SG Off Voltage
●
●
10% to 90%/90% to 10%
35
11
●
●
10% to 90%/90% to 10%
SG Rise/Fall Times
tSG-BG
SG to BG Enable Lag Time
4V On/Off Thresholds
tTG-BG
TG to BG Enable Lag Time
4V On/Off Thresholds
11
10% to 90%/90% to 10%
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Supply current specification does not include external FET gate
charge currents. Actual supply currents will be higher and vary with
operating frequency, operating voltages and the type of external switch
elements used. See Applications Information.
0.5
35
●
●
tSGr/f
11.5
0.1
ns
11.5
0.1
ns
0.5
35
80
●
150
V
V
V
V
ns
300
100
ns
ns
Note 3: Guaranteed but not tested.
Note 4: The LT1681E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the – 40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. For guaranteed performance to
specifications over the –40°C to 85°C range, the LT1681I is available.
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TYPICAL PERFOR A CE CHARACTERISTICS
ICC Supply Current
vs Temperature
ICC Supply Current
vs SHDN Pin Voltage
1100
20
17
16
15
–55 –40
ICC SUPPLY CURRENT (mA)
ICC SUPPLY CURRENT (nA)
ICC SUPPLY CURRENT (mA)
18
18
TA = 25°C
VCC = 12V
19
ICC Supply Current
vs VCC Supply Voltage
900
700
500
0
40
80
TEMPERATURE (°C)
125
1681 G01
0
100
200
300
400
SHDN PIN VOLTAGE (mV)
500
1681 G02
TA = 25°C
17
16
15
9
10
12
14
16
SUPPLY VOLTAGE (V)
18
1681 G03
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LT1681
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TYPICAL PERFOR A CE CHARACTERISTICS
IBST Boost Supply Current
vs Temperature
ICC Supply Current
vs SHDN Pin Voltage
UVLO ICC Supply Current
vs Temperature
60
5.2
1
5.1
5.0
4.9
4.8
–55 –40
0
40
80
TEMPERATURE (°C)
UVLO ICC SUPPLY CURRENT (mA)
ICC SUPPLY CURRENT (µA)
IBST BOOST SUPPLY CURRENT (mA)
TA = 25°C
40
20
0
125
0
0.2
0.4
0.6
0.8
1.0
SHDN PIN CURRENT (V)
1681 G04
5.00
4.95
125
1.260
50
40
30
–55 –40
0
40
80
TEMPERATURE (°C)
0
40
80
TEMPERATURE (°C)
125
1681 G10
1.245
1.240
–55 –40
0
40
80
TEMPERATURE (°C)
125
1681 G09
Soft-Start Output Current
vs Soft-Start Pin Voltage
60
TA = 25°C
VSS = 2V
SOFT-START OUTPUT CURRENT (µA)
12
SOFT-START OUTPUT CURRENT (µA)
VC PIN SHORT-CIRCUIT CURRENT LIMIT (mA)
10
–55 –40
1.250
Soft-Start Output Current
vs Temperature
25
15
125
1.255
1681 G08
VC Pin Short-Circuit Current Limit
vs Temperature
125
Error Amp Reference
vs Temperature
60
1681 G07
20
0
40
80
TEMPERATURE (°C)
1681 G06
ERROR AMP REFERENCE (V)
5VREF SHORT-CIRCUIT CURRENT LIMIT (mA)
5VREF VOLTAGE (V)
5.05
40
80
TEMPERATURE (°C)
0.5
–55 –40
1.2
5VREF Short-Circuit Current Limit
vs Temperature
5.10
0
0.6
1681 G05
5VREF Voltage vs Temperature
4.90
–55 –40
0.8
11
10
9
8
–55 –40
40
20
0
0
40
80
TEMPERATURE (°C)
125
1681 G11
0
100
200
300
400
SOFT-START PIN VOLTAGE (mV)
500
1681 G12
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LT1681
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TYPICAL PERFOR A CE CHARACTERISTICS
Soft-Start Output Current
vs Soft-Start Pin Voltage
Current Sense Amplifier
Bandwidth vs Temperature
60
8
CURRENT SENSE AMP BANDWIDTH (MHz)
SOFT-START OUTPUT CURRENT (µA)
TA = 25°C
40
20
0
0
1
2
3
4
SOFT-START PIN VOLTAGE (V)
5
1681 G13
7
6
5
4
3
2
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
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SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding
positive-going threshold of 1.25V enables the LT1681.
150mV of input hysteresis resists mode switching instability.
The SHDN pin can be controlled by either a logic-level
input or with an analog signal. This shutdown feature is
typically used for input supply undervoltage protection. A
resistor divider from the converter input supply to the
SHDN pin monitors that supply for control of system
power-up sequencing, etc. All internal functions are disabled during shutdown.
OVLO (Pin 2): Overvoltage Shutdown Sense. Typically
connected to input supply through a resistor divider. If pin
voltage exceeds 1.25V, the LT1681 switching function is
disabled to protect boosted circuitry from exceeding absolute maximum voltage. 40mV of input hysteresis resists
mode switching instability. Exceeding the OVLO threshold
also triggers soft-start reset, resulting in a graceful recovery from an input transient event.
THERM (Pin 3): System Thermal Shutdown. Auxiliary
shutdown pin that is typically used for system thermal
protection. If pin voltage exceeds 1.25V, the LT1681
switching function is disabled. 40mV of input hysteresis
resists mode switching instability. Exceeding the THERM
threshold also triggers soft-start reset, resulting in a
graceful recovery.
SGND (Pin 4): Signal Ground Reference. Careful board
layout techniques must be used to prevent corruption of
the signal ground reference. High current switching paths
must be oriented on the converter ground plane such that
currents to/from the switches do not affect the integrity of
the LT1681 signal ground reference.
5VREF (Pin 5): 5V Local Reference. Allows connection of
external loads up to 20mA DC. Typically bypassed with
1µF ceramic capacitor to SGND. Reference output is
current limit protected to a typical value of 45mA. If the
load on the 5V reference exceeds the current limit value,
LT1681 switching function is disabled and the soft-start
function is reset.
FSET (Pin 6): Oscillator Timing Pin. Connect a resistor
(RFSET) from the 5VREF pin to this pin and a capacitor
(CFSET) from this pin to ground.
The LT1681 oscillator operates by monitoring the voltage
on CFSET as it is charged via RFSET. When the voltage on the
FSET pin reaches 2.5V, the oscillator rapidly discharges
the capacitor with an average current of 0.8mA. Once the
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LT1681
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voltage on the pin is reduced to 1.5V, the pin becomes high
impedance and the charging cycle repeats. The oscillator
operates at twice the switching frequency of the controller.
Oscillator frequency fOSC can be approximated by the
relation:
fOSC
–1
R
2
FSET
–
6
–
4
≅ 0.5 • 10 + C FSET
+ 8 • 10 +
R
3
FSET
–1
SYNC (Pin 7): Oscillator Synchronization Input Pin with
TTL-Level Compatible Input. The SYNC input signal (at the
desired synchronized operating frequency) controls both
the internal oscillator (running at twice the SYNC frequency) and the output switch phase. If the synchronization function is not desired, this pin may be shorted to
ground.
The LT1681 internal oscillator drives a toggle flip-flop that
assures ≤ 50% duty cycle operation during oscillator freerun. The oscillator, therefore, runs at twice the operating
frequency of the converter. The SYNC input decoder
incorporates a frequency doubling circuit for oscillator
synchronization, resetting the internal oscillator on both
the rising and falling edges of the input signal.
The SYNC input decoder also differentiates transition
phase and forces the toggle flip-flop to phase-lock with the
SYNC input. A transition to logic high on the SYNC input
signal corresponds to the initiation of a new switching
cycle (primary switches turning on pending current control) and a transition to logic low forces a primary switch
off state. As such, the maximum operating duty cycle is
equal to the duty cycle of the SYNC signal. The SYNC input
can therefore be used to reduce the maximum duty cycle
of the converter by reducing the duty cycle of the SYNC
input.
SS (Pin 8): Soft-Start. Connect a capacitor (CSS) from this
pin to ground.
The output voltage of the LT1681 error amplifier corresponds to the peak current sense amplifier output detected before resetting the switch outputs. The soft-start
circuit forces the error amplifier output to a zero sense
current for start-up. A 10µA current is forced from this pin
onto an external capacitor. As the SS pin voltage ramps
up, so does the LT1681 internally sensed current limit.
This effectively forces the internal current limit to ramp
from zero, allowing overall converter current to slowly
increase until normal output regulation is achieved. This
function reduces output overshoot on converter start-up.
The soft-start function incorporates a 1VBE “dead zone”
such that a zero current condition is maintained on the V C
pin until the SS pin rises to 1VBE above ground.
The SS pin voltage is reset to start-up condition during
shutdown, undervoltage lockout and overvoltage or
overcurrent events, yielding a graceful converter output
recovery from these events.
VFB (Pin 9): Error Amplifier Inverting Input. Typically
connected to a resistor divider from the output and compensation components to the VC pin.
The VFB pin is the converter output voltage feedback node.
Input bias current of ~50nA forces the pin high in the event
of an open-feedback path condition. The error amplifier is
internally referenced to 1.25V.
Values for the VOUT to VFB feedback resistor (RFB1) and the
VFB to ground resistor (RFB2) can be calculated to program
converter output voltage (VOUT) via the following relation:
VOUT = 1.25 • (RFB1 + RFB2)/RFB2
VC (Pin 10): Error Amplifier Output. The LT1681 error
amplifier is a low impedance output inverting gain stage.
The amplifier has ample current source capability to allow
easy integration of isolation optocouplers that require bias
currents up to 10mA. External DC loading of the VC pin
reduces the external current sourcing capacity of the
5VREF pin by the same amount as the load on the VC pin.
The error amplifier is typically configured using a feedback
RC network to realize an integrator circuit. This circuit
creates the dominant pole for the converter regulation
feedback loop. Integrator characteristics are dominated
by the value of the capacitor connected from the VC pin to
the VFB pin and the feedback resistor connected to the VFB
pin. Specific integrator characteristics can be configured
to optimize transient response.
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LT1681
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The error amplifier can also be configured as a
transimpedance amplifier for use in secondary-side controller applications. (See Applications Information section
for configuration and compensation details)
SENSE (Pin 11): Current Sense Amplifier (CSA) Noninverting Input. Current is monitored via a ground referenced current sense resistor, typically in series with the
source of the bottom-side switch FET. Internal limit circuitry provides for a maximum peak value of 150mV across
the sense resistor during normal operation.
IMAX (Pin 12): Primary Current Runaway Protection. The
IMAX pin is used to detect primary-side switch currents
and shuts down the primary switches if a current runaway
condition is detected. The IMAX function is not disabled
during the current sense blanking interval. The pin is
typically connected to the primary bottom-side switch
source and monitors switch current via a ground-referenced current sense resistor. If the pin voltage exceeds
360mV, LT1681 switching function is disabled in 130ns.
Exceeding the IMAX threshold also triggers a soft-start
reset, resulting in a graceful recovery from a current
runaway event. For single-sense resistor systems, this pin
can be shorted to SENSE for protection during the blanking interval or shorted to SGND if not used.
SG (Pin 13): Synchronous Switch Output Driver. This pin
can be connected directly to the gate of the synchronous
switch if small FETs are used (CGATE < 5000pF), however,
the use of a gate drive buffer is recommended for peak
efficiencies.
The SG pin output is synchronized and out-of-phase with
the BG output. The control timing of the SG output causes
its transition to “lead” the primary switch path during turnon by 150ns.
VCC (Pin 14): IC Local Power Supply Input. Bypass with a
capacitor at least 10 times greater than C5VREF to PGND.
The LT1681 incorporates undervoltage lockout that disables switching functions if VCC is below 8.4V. The LT1681
supports operational VCC power supply voltages from 9V
to 18V (20V absolute maximum).
PWRGND (Pin 15): Output Driver Ground Reference.
Connect through low impedance trace to VIN decoupling
capacitor.
BG (Pin 16): Bottom-Side Primary Switch/Forward Switch
Output Driver. Pin can be connected directly to gate of
primary bottom-side and forward switches if small FETs
are used (CGATE total < 5000pF), however, the use of a gate
drive buffer is recommended for peak efficiencies.
The BG output is enabled at the start of each oscillator
cycle in phase with the TG pin but is timed to “lag” the TG
output during turn-on and “lead” the TG output during
turn-off. These delays force the concentration of transitional losses onto the bottom-side primary switch.
BLKSENS (Pin 17): Blanking Sense Input. The current
sense function (via SENSE pin) is disabled while the
BLKSENS pin is below 5V. BLKSENS is typically connected to the gate of the bottom-side primary switch
MOSFET.
BSTREF (Pin 18): VBST Supply Reference. Typically connects to source of topside external power FET switch.
TG (Pin 19): Topside (Boosted) Primary Output Driver. Pin
can be connected directly to gate of primary topside
switch if small FETs are used (CGATE < 5000pF), however,
the use of a gate drive buffer is recommended for peak
efficiencies.
VBST (Pin 20): Topside Primary Driver Bootstrapped Supply. This “boosted” supply rail is referenced to the BSTREF
pin.
Supply voltage is maintained by a bootstrap capacitor tied
from the VBST pin to the boosted supply reference (BSTREF)
pin. The charge on the capacitor is refreshed each switch
cycle through a Schottky diode connected from the VCC
supply (cathode) to the VBST pin (anode). The bootstrap
capacitor (CBOOST) must be at least 100 times greater than
the total load capacitance on the TG pin. A capacitor in the
range of 0.1µF to 1µF is generally adequate for most
applications. The bootstrap diode must have a reversebreakdown voltage greater than the converter VIN. The
LT1681 supports operational VBST supply voltages up to
90V (absolute maximum) referenced to ground.
Undervoltage lockout disables the topside switch until
VBST-BSTREF > 7.0V for start-up protection of the topside
switch.
1681f
8
×4
ILIM
1.25V
Q
T
–
+
SGND 4
OVLO 2
–
+
REFERENCE
GENERATOR
1.25V
UVL
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