TI UC1874

UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
High Efficiency, Synchronous, Step-down (Buck) Controllers
FEATURES
DESCRIPTION
•
Operation to 36V Input Voltage
•
Fixed Frequency Average Current
Mode Control
•
Standby Mode for Improved
Efficiency at Light Load
•
Drives External N-Channel
MOSFETs for Highest Efficiency
•
Sleep Mode Current < 50mA
•
Complementary 1 Amp Outputs with
Regulated Gate Drive Voltage
•
LDO (Low Drop Out) Virtual 100%
Duty Cycle Operation
The UC3874 family of synchronous step-down (Buck) regulators provides
high efficiency power conversion from an input voltage range of 4.5 to 36
volts. The UC3874 is tailored for battery powered applications such as
laptop computers, consumer products, communications systems, and
aerospace which demand high performance and long battery life. The
synchronous regulator replaces the catch diode in the standard buck
regulator with a low Rds(on) N-channel MOSFET switch allowing for
significant efficiency improvements. The high side N-channel MOSFET
switch is driven out of phase from the low side N-channel MOSFET switch
by an on-chip bootstrap circuit which requires only a single external
capacitor to develop the regulated gate drive. Fixed frequency, average
current mode control provides the regulator with inherent slope
compensation, tight regulation of the output voltage, and superior load and
line transient response. Switching frequencies up to 300kHz are possible.
•
Non-Overlapping Gate Drives
BLOCK DIAGRAM
Not Recommended for New Designs.
Light load efficiency is improved by a fully programmable standby mode, in
which the quiescent current consumption of the controller is significantly reduced. The reduction is achieved by disabling the MOSFET driver outputs
and the internal oscillator when the controller has sensed that the the output load current has dropped a user programmable amount from full load.
(continued)
UDG-95005-1
2/98
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
During standby operation, the output capacitor supplies
all of the load current requirements. Normal operation returns when the output voltage has drooped by 1%. Reverse current in the inductor is prevented by on-chip
circuitry providing additional efficiency improvements.
Virtual 100% duty cycle operation is easily attained by
the controller even though a bootstrapped high side drive
technique is employed.
UC3874-1 is designed for logic level MOSFETs and has
UVLO turn-on and turn-off thresholds of 4.5V and 4.4V
respectively. The UC3874-2 is designed for standard
power MOSFETs and has UVLO turn-on and turn-off
thresholds of 10V and 9V respectively. A precision 2.5V
reference can supply 20mA to external circuitry. An error
amplifier with soft start, high bandwidth current amplifier,
and a synchronizable oscillator are additional features.
A low power sleep mode can be invoked through the SS
pin. Quiescent supply current in sleep mode is typically
less than 50mA. Two UVLO options are available. The
Available packages include 18-pin plastic and ceramic
DIP (N, J), 18-pin SOIC (DW), and 20-pin plastic and
ceramic leadless chip carriers (Q, L).
CONNECTION DIAGRAMS
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36V
Boost Voltage (BOOT). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50V
OUTPUT Drivers (HDRIVE, LDRIVE) Currents
(continuous) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±0.25A
(peak) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±1A
VREF Current . . . . . . . . . . . . . . . . . . . . . . . . Internally Limited
Inputs (VSNS, SS, COMP, CT) . . . . . . . . . . . . . . . . -0.3 to 10V
Inputs (ISNS+, ISNS-) . . . . . . . . . . . . . . . . . . . . . . . -0.3 to 20V
Outputs (CAOUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to10V
Soft start Sinking Current . . . . . . . . . . . . . . . . . . . . . . . . 1.5mA
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300°C
DIL-18 (TOP VIEW)
J or N, DW Packages
All currents are positive into, negative out of the specified terminal. All voltages are referenced to GND. Consult Packaging
Section of Databook for thermal limitations and considerations
of packages.
PLCC-20 (TOP VIEW)
Q Package
LCC-28 (TOP VIEW)
L Package
2
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
ELECTRICAL CHARACTERISTICS: Unless otherwise stated these specifications apply for TA = –55°C to +125°C for
UC1874; TA = –25°C to +85°C for UC2874; 0°C to +70°C for UC3874; VCC = 12V, Ct = 680pF, CCAP = 1µF; CBOOT = 0.1µF;
TA = TJ.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX UNITS
Overall Section
Supply Current, Sleep
SOFTSTART = 0V; TA=25°C
Supply Current, Operating
Supply Current, Standby
UC2874-1, -2, UC3874-1, -2
Supply Current, Standby
UC1874-1, -2
VCC Turn-on Threshold
VCC Turn-off Threshold
30
75
µA
8.5
12
mA
2.5
3.5
mA
5.5
mA
UCX874-2
10
10.5
V
UCX874-1
4.5
4.8
V
UCX874-2
8.5
9
V
UCX874-1
4.1
4.4
V
TA = 25°C
1.97
2
Voltage Amplifier Section
Input Voltage
VSNS Bias Current
2.03
V
–500
25
500
nA
Transconductance
ICOMp = +10µA to –10µA, UC3874 -1, -2;
UC2874-1, -2
400
675
1000
µMho
Transconductance
ICOMp = +5µA to –5µA, UC1874-1,-2
250
675
1250
µMho
2.9
3.1
3.25
V
1.85
V
VOUT High
VOUT Low
Output Source Current
SB = VREF
VOUT = 1V; UC3874-1,-2; UC2874-1,-2
10
35
VOUT = 1V; UC1874 -1,-2
5
35
–6
0
µA
µA
Current Amplifier Section
Input Offset Voltage
VCOMP = 2.5V
Input Bias Current (SENSE)
VCM = 2.5V
Open Loop Gain
VCM = 2.5V, VOUT = 1V to 3.5V
80
110
dB
VOUT High
RCAOUT = 100k to GND, TA = 25°C
3.6
3.7
V
0.7
0.86
V
100
120
µA
–500
6
mV
500
nA
VOUT Low
RCAOUT = 100k to VREF, TA = 25°C
Output Source Current
VOUT = 0V, TA = 25°C
80
Common Mode REJ Ratio
VCM = 2V to 3V
70
90
dB
Gain Bandwidth Product
FIN = 100kHz, 10mV p-p
2
3.5
MHz
Reference Section
Output Voltage
IREF = 0mA, TA = 25°C
2.462
2.5
2.538
V
IREF = 0mA
2.437
2.5
2.563
V
Load Regulation
IREF = 0mA to 5mA
2
± 15
mV
Line Regulation
VCC = 12V to 24V
2
± 15
mV
Short Circuit Current
VREF = 0V
10
20
25
mA
Initial Accuracy
TA = 25°C
85
100
115
kHz
Voltage Stability
VCC = 12V to 18V
1
1.5
%
Total Variation
Line, Temperature
120
kHz
Ramp Amplitude (p-p)
TA = 25°C
2.48
2.7
2.85
V
Ramp Valley Voltage
TA = 25°C
0.86
0.95
Sleep Threshold
Measured on SS, TA = 25°C
0.25
0.6
0.8
V
SS Charge Current
VSS = 2.5V
4
6
10
µA
Oscillator Section
80
V
Sleep/Soft Start/Bootstrap Section
3
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
ELECTRICAL CHARACTERISTICS: Unless otherwise stated these specifications apply for TA = –55°C to +125°C for
UC1874; TA = –25°C to +85°C for UC2874; 0°C to +70°C for UC3874; VCC = 12V, Ct = 680pF, CCAP = 1µF; CBOOT = 0.1µF;
TA = TJ.
PARAMETER
MIN
TYP
0.5
0.8
9.5
10.2
UCX874-1, Low Driver ON
6
7.5
9
V
UCX874-2, VCAOUT > VCTPEAK
7
8
9
V
UCX874-1, VCAOUT > VCTPEAK
2.7
3.5
4
V
Output High Voltage
IOUT = –50mA, Boot = 23V
21
22.2
Output Low Voltage
IOUT = 50mA
1
2.2
V
SS Discharge Current
TEST CONDITIONS
VSS = 2.5V
MAX UNITS
mA
Sleep/Soft start/Bootstrap Section (continued)
Bootstrap Regulation Voltage
Bootstrap Refresh Voltage
UCX874-2, Low Driver ON
12.5
V
High Side Driver Output Section
V
IOUT = 10mA
300
500
mV
Output Low (UVLO)
IOUT = 50mA, VCC = 0V
0.9
1.5
V
Output Rise Time
COUT = 1nF
40
160
ns
Output Fall Time
COUT = 1nF
30
100
ns
Low Side Driver Output Section
Output High Voltage
IOUT = –50mA, VCAP = 11V
Output Low Voltage
IOUT = 50mA
8.8
9.5
1
2.2
V
V
IOUT = 10mA
300
500
mV
Output Low (UVLO)
IOUT = 50mA, VCC = 0V
0.9
1.5
V
Output Rise/Fall Time
CLOAD = 1nF
40
160
ns
Output Fall Time
COUT = 1nF
30
100
ns
9.2
9.8
10.4
V/V
X10 Amplifier Section
Gain
VISNS ± VISNS = 20mV to 80mV
Slew Rate Rising
TA = 25°C
1
1.4
V/µs
Slew Rate Falling
TA = 25°C
2
3.5
V/µs
Input Resistance
TA = 25°C
60
100
165
kΩ
PIN DESCRIPTIONS
the low side driver to cycle itself on for the few cycles
required to replenish CBST. In this way, virtual 100% duty
cycle operation is provided.
BOOT: This pin provides the high side rail for the
HDRIVE output.
An external capacitor (CBST) is
connected between this pin and the drain of the external
low side MOSFET. When the low side MOSFET is
conducting Cbst is charged to 11V via an external diode
tied to CAP. When the low side MOSFET turns off and
the high side MOSFET turns on, the Cbst bootstraps
itself up with the source of high side MOSFET, ultimately
providing a 10V Vgs for the upper MOSFET. Since this
10V is referenced to the source of the high side
N-channel MOSFET, the actual voltage on BOOT and
HDRIVE is approximately 10V above VCC while the high
side MOSFET is conducting. The voltage on BOOT is
continuously monitored during low input voltage
conditions when the duty cycle equals approximately
100% to insure that a sufficient gate drive level is being
supplied by the UC3874. If the voltage on BOOT falls
below 8V (UC3874-2) or 3.5V (UC3874-1), the IC forces
CA-: This is the inverting input to the current amplifier.
Connect a series resistor and capacitor between this pin
and CAOUT to set the current loop compensation. An input resistor between this pin and ISOUT provides the inductor current sense signal to the amplifier and also
sets the high frequency gain of the amplifier. The common mode operating range for this input is between GND
and 4V. The normal range during operation is between
2V and 3V.
CAOUT: This is the output of the wide bandwidth current
amplifier and one of the inputs to the PWM duty cycle
comparator. The output signal generated by this amplifier
commands the PWM to force the correct duty cycle to
maintain output voltage in regulation. The output can
swing from 0.1V to 4V.
4
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
CAP: A capacitor is normally connected between this pin
and GND providing bypass for the internal 11V regulator.
Charge is transferred from this capacitor to CBST via an
external diode when the low side MOSFET is conducting.
If VCC ≤ 10V logic level MOSFETs are generally specified. CAP should then be shorted to VCC in conjunction
with a low VF Schottky to BOOT to maximize the gate
drive amplitude. This technique provides adequate gate
drive signal amplitudes with VCC as low as 4.5V. For
high input voltage applications, a simple external shunt
zener regulator circuit can be connected to CAP, thereby
offloading power dissipation requirements from the IC to
an external transistor.
ISNS-: This is the inverting input to the X10 instrumentation amplifier. The common mode input range for this pin
extends from GND to VCC. A low value resistor in series
with the output inductor is connected between this pin
and ISNS+ to develop the current sense signal.
COMP: This is the output of the voltage amplifier. It provides the current command signal to the current amplifier. The voltage is clamped to approximately 3.2V.
PGND: This is the high current ground for the IC. The
MOSFET driver transistors are referenced to this ground.
For best performance an external star ground connection
should be made between this pin, the source of the low
side MOSFET, the capacitor on CAP, the anodes of any
external Schottky clamp diodes and the output filter capacitor. As with all high frequency layouts, a ground
plane and short leads are highly recommended.
ISNS+: This is the non-inverting input to the X10 instrumentation amplifier. The common mode input range for
this pin extends from GND to VCC.
ISOUT: This is the output of the X10 instrumentation amplifier. The output voltage on this pin is level shifted 2V
above GND, such that if a 100mV differential input is applied across ISNS+ and ISNS-, the output will be 3V.
CT: A capacitor from CT to GND sets the PWM oscillator
frequency according to the following equation:
F=
1
14250 • CT
SB: The voltage on SB sets the output current level at
which standby mode is initiated. A voltage level from 0V
to 1V programs the threshold from 50% to 0% of full load
current. Full load current corresponds to a 100mV differential signal across the ISNS inputs. Since this is a
high impedance input, a voltage divider derived from
VREF may be used to program this level. Another possible use is to actively control this level with external circuitry to adaptively control converter efficiency. Tying SB
to VREF disables standby mode operation.
Use a high quality ceramic capacitor with low ESL and
ESR for best results. A minimum CT value of 220pF insures good accuracy and less susceptibility to circuit layout parasitics. The oscillator and PWM are designed to
provide practical operation to 300kHZ.
GND: All voltages are measured with respect to this pin.
All bypass capacitors and timing components except
those listed under the PGND pin description should be
connected to this pin. Component leads should be as
short and direct as possible.
SS: A capacitor from this pin to GND in conjunction with
an internal 10mA current source provides a soft start
function for the IC. The voltage level on SS clamps the
output of the voltage amplifier through an internal buffer,
thus providing a controlled startup. The SS time is approximately:
HDRIVE, LDRIVE: The outputs of the PWM are totem
pole MOSFET gate drivers on the HDRIVE and LDRIVE
pins. The outputs can sink approximately 1A and source
500mA. This characteristic optimizes the switching transitions by providing a controlled dV/dT at turn-on and a
lower impedance at turn-off. These are complementary
outputs with a typical deadtime of 200ns. Internal circuitry prevents the possibility of simultaneous conduction
of the output MOSFETs (shoot through). HDRIVE is the
high side bootstrapped output. Its upper power supply rail
is the BOOT pin which means that its output will fly approximately 10V above VCC when the upper side of the
totem pole output is conducting. The power supply rail for
LDRIVE is CAP. As a result the Vgs of both gates are
regulated to approximately 10V if VCC is >11V. A series
resistor between these pins and the MOSFET gates of at
least 10 ohms can be used to control ringing. Additionally, a low VF Schottky diode should be connected between these pins and GND to prevent substrate
conduction and possible erratic operation.
VO
• 3V
VIN
10µ A
CSS •
Once the device has completed its soft start cycle, a low
power sleep mode can be invoked by pulling SS below
0.5V typically. In sleep mode, all of the device functions
are disabled except for those which are required to bring
the device out of sleep mode when SS is released. Typical sleep mode supply current is less than 50mA.
VCC: Positive supply rail for the IC. Bypass this pin to
GND with a 1mF low ESL/ESR ceramic capacitor. The
maximum voltage for VCC is 36V. The turn on voltage
level on VCC is 4.5V with 100mV of hysteresis for the
5
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
UC3874-1 and 10V with 1V of hysteresis for the
UC3874-2.
PWM stage until VREF has attained approximately 90%
of its nominal value. Bypass VREF to GND with a 0.1mF
ceramic capacitor for best performance.
VREF: VREF is the output of the precision reference.
The output is capable of supplying 20mA to peripheral
circuitry and is internally short circuit current limited.
VREF is disabled and low whenever VCC is below the
UVLO threshold, and when SS is pulled below 0.5V. A
VREF “good” comparator senses VREF and disables the
VSNS: This pin is the inverting input of the voltage amplifier and serves as the output voltage feedback point for
the synchronous regulator. It senses the output voltage
through a voltage divider which produces a nominal 2.0V.
APPLICATION DIAGRAM
UDG-95006-1
OPERATION: Refer to 5V, 25W Application Schematic
The UC3874 employs a fixed frequency average current
mode control buck topology to convert a higher battery
voltage down to a tightly regulated output voltage. Special design techniques allow this bipolar IC to deliver exceptional performance while consuming approximately
6mA of supply current over an input voltage range of 4.5
to 35 volts. Fixed frequency operation allows synchronization to an existing system clock, and easier filtering.
Average current mode control provides inherent slope
compensation and accurate short circuit current limiting.
An additional benefit is its ability to maintain a constant
regulator gain regardless of whether the inductor current
is continuous or discontinuous.
The output inductor current is sensed by an external low
value shunt resistor (Rsense). This signal at full load
current should be no larger than 100mV in order to minimize sensing losses. The differential voltage across
Rsense is amplified by the internal X10 instrumentation
amplifier. The common mode input range for this amplifier extends from GND to VCC in order to maintain accurate current sensing under normal conditions as well as
abnormal conditions such as output short circuit and low
drop out (LDO) modes. The output of the X10 instrumentation amplifier is applied to the inverting input of the current amplifier through an external resistor. The
converter’s output voltage feedback is applied to the
6
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
VSNS pin through an external voltage divider. The difference between the voltage at VSNS and the internal reference level at the non-inverting input is amplified by the
voltage amplifier and applied to the non-inverting input of
the current amplifier. This instantaneous reference level
forms the current command input for the average current
control loop. The average current amplifier develops the
duty cycle command signal by integrating the current
feedback signal with respect to the instantaneous current
command input. This output is compared to the fixed high
amplitude oscillator ramp waveform at the inputs of the
PWM comparator to develop duty cycle information for
the PWM drive. The large amplitude oscillator ramp provides both high noise margin and built-in slope compensation in average current mode control methodology. The
fixed frequency oscillator is programmed with a single external capacitor connected between CT and GND, and is
capable of switching frequencies up to 300kHz. The
UC3874 can be synchronized to an external clock by capacitively coupling the signal to the junction of the capacitor at CT and a low value resistor tied to GND. Refer
to Application Note U-111.
effective duty cycle dependent only upon the value of
CBST.
High efficiency is obtained primarily by the low side
MOSFET which replaces the Schottky diode in the standard buck configuration. Its low Rds(on) produces a
much lower voltage drop than a low VF Schottky diode.
As output voltages get lower, these improvements become more evident. Additional efficiency improvements
at light regulator load currents are obtained by automatic
switchover to standby mode. In standby mode, the
UC3874 disables its MOSFET drivers and oscillator saving both quiescent supply current consumption and more
importantly MOSFET gate drive charge current. Standby
mode is initiated when the output inductor current has
dropped to a user programmable fraction of the designed
full load current. Programmability is easily attained by
setting the SB pin to a voltage level between 0V and 1V,
which corresponds to 50% to 0% of the peak load current. In this manner, the user can accurately determine
when standby mode is initiated, giving the flexibility to directly shape the efficiency vs. load curve. In standby
mode, all output current requirements are handled by the
output capacitor. Since the output capacitor isn’t being
refreshed by the PWM converter, the output voltage will
decay at a rate determined by the load current and the
output capacitor value:
The PWM drive signal is applied to the complementary
output driver stages. Since the high side switch is an Nchannel MOSFET, a means for driving its gate above
VCC is required. This is accomplished via the internal
11V regulator and an external capacitor (CBST). CBST is
charged through an external diode to VCC or CAP when
the low side MOSFET is on. The charging level on Cbst
is internally regulated to 11V minus an external diode
drop by the UC3874 as long as VCC is above 11V. When
the low side MOSFET turns off, CBST is applied across
the gate to the source of the upper MOSFET allowing it
to begin turn-on. As the upper MOSFET turns on, it lifts
or bootstraps the low end of CBST, along with its source.
Shortly thereafter, the source voltage level is reduced by
RDS(on) · Iload below VCC. When VCC < 10V, Vgs for
the high side MOSFET is approximately equal to VCC. If
VCC < 8V, logic level MOSFETs are recommended. In
these applications, CAP should be shorted to VCC and
an external Schottky diode is connected between
CAP/VCC and BOOT. For low battery applications, a synchronous regulator must be capable of LDO or 100%
duty cycle operation. The UC3874 includes circuitry to
insure that this mode of operation is possible even
though it uses a bootstrapped drive technique for the
high side MOSFET. During commanded 100% duty cycle
operation, the UC3874 monitors the Vgs drive signal applied to the high side MOSFET, and automatically provides complementary pulses to refresh the bootstrap
capacitor when this voltage falls below a set threshold. In
this way, near 100% duty cycle operation is possible, with
d V ILOAD
=
d T COUT
Normal operation returns when the output voltage has
decayed by approximately 1% from its nominal value.
Standby operation can be easily disabled by connecting
the SB pin directly to VREF. Another efficiency consideration is the the possibility of reverse current in the output inductor. For a non-synchronous regulator this isn’t a
problem since the diode will block reverse current, allowing discontinuous inductor current operation at light
loads. Since the synchronous regulator replaces the diode with a switch, reverse current can and will flow if the
low side switch is on when the inductor is depleted. The
UC3874 includes circuitry to prevent reverse current from
flowing in the inductor by disabling the low side gate
drive signal during discontinuous mode operation. This
increases efficiency by eliminating unnecessary I2R
losses in the MOSFET and the inductor.
Soft start is recommended for Buck converters to reduce
stress on the power components during startup, and to
reduce overshoot of the output voltage. This improves
reliability. The UC3874 includes a user programmable
soft start pin to implement this feature. An internal 10mA
current source charges the external soft start capacitor
which provides a clamp at the output of the voltage
7
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
amplifier. An ultra low power sleep mode is also invoked
from the SS pin. A voltage level below 0.5V on this pin
reduces total standby current to less than 50mA. Short
circuit protection is inherent to the average current mode
technique with proper compensation of the current
amplifier. To prevent operation of the MOSFETs with an
inadequete drive signal, an undervoltage lockout circuit
suppresses the output drivers until the input supply
voltage is sufficiently high enough for proper operation.
The UC3874-1 is intended for applications with logic level
MOSFETs and its VCC turn-on and turn-off thresholds
are 4.5V/4.4V respectively. The UC3874-2 is intended for
applications with standard MOSFETs and has UVLO
turn-on and turn-off thresholds of 10V and 9V
respectively. The precision 2.5V reference can provide
20mA to power external circuitry. The reference output is
disabled during UVLO and sleep modes.
ORDERING INFORMATION
UVLO Turn
On/Off
Threshold
1: 4.5V/4.4V
2: 10V/9V
Temperature
Range
Package
J: Ceramic DIL-18
N: Plastic DIL-18
DW: SOIC-18
1: –55°C to +125°C
2: –40°C to +85°C
3: 0°C to +70°C
TYPICAL PERFORMANCE INFORMATION
2.05
11
2.04
10
2.03
2.02
VOLTS
ICC (mA)
9
8
7
2.01
2
1.99
1.98
6
1.97
5
1.96
4
1.95
-75
-25
25
75
125
-50
0
25
50
75
100
TEMPERATURE °C
TEMPERATURE °C
Figure 1. Supply Current
Figure 2. VAMP Input Voltage
140
760
130
720
FREQUENCY (KHz)
TRANSCONDUCTANCE (uMho)
-25
680
640
600
120
110
100
90
80
560
-50
-25
0
25
50
75
-75
100
-25
25
75
TEMPERATURE °C
TEMPERATURE °C
Figure 3. Volt Amp GM (IOUT = ± 10µA)
Figure 4. Oscillator Frequency vs. Temperature
(CT = 680pF)
8
125
UC1874-1,-2
UC2874-1,-2
UC3874-1,-2
PERFORMANCE INFORMATION (continued)
0
70
ROOM
105
0.16
% DUTY CYCLE
100
VSC (V)
0.14
0.12
95
90
85
0.1
80
0.08
75
0
1
2
3
4
5
-75
VCM (V)
-25
25
75
TEMPERATURE °C
Figure 5. Short Circuit Limit Voltage Reflected to
Input of Current Amp vs. Current Amp Common
Mode Voltage
Figure 6. High Drive Maximum Duty Cycle
(UC1874-1,-2)
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 • FAX (603) 424-3460
9
125
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