UNITRODE UC2578, UC3578 Technical data

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UNITRODE UC2578, UC3578 Technical data

UC2578

application

INFO

available

UC2578

UC3578

Buck Pulse Width Modulator Stepdown Voltage Regulator

FEATURES

Provides Simple Single Inductor Buck PWM Step-Down Voltage Regulation

Drives External High Side NMOS Switch

14V to 72V Input Voltage Operating Range

Contains 100kHz Internal Oscillator, 2V Reference and UVLO

Soft Start on Power Up

Overcurrent Shutdown Followed by Soft Start

DESCRIPTION

The UC3578 is a PWM controller with an integrated high side floating gate driver. It is used in buck step down converters and regulates a positive output voltage. Intended to be used in a distributed power system, the IC allows operation from 14V to 72V input voltage which range includes the prevalent telecomm bus voltages. The output duty cycle of the UC3578 can vary between 0% and 90% for operation over the wide input voltage and load conditions.

The UC3578 simplifies the design of the single switch PWM buck converter by incorporating a floating high side driver for an external N-channel MOSFET switch. It also features a 100kHz fixed frequency oscillator, an internal 2V precision reference, an error amplifier configured for voltage mode operation, and a PWM comparator with latching logic. Complementing the traditional voltage mode control block, the UC3578 incorporates an overcurrent shutdown circuit with full cycle soft re-start to limit the input current to a user defined maximum value during overload operation. Additional functions include an under voltage lockout circuit to insure that sufficient input supply voltage is present before any switching activity can occur.

The UC2578 and the UC3578 are both available in surface mount and thru-hole power packages.

ORDERING INFORMATION

TYPICAL APPLICATION DIAGRAM

 

TEMPERATURE RANGE

PACKAGE

UC2578DP

–40°C to +85°C

Power SOIC

UC2578N

 

Power PDIP

UC3578DP

0°C to +70°C

Power SOIC

UC3578N

 

Power PDIP

15-40 VIN

RSENSE

 

 

 

IRFZ34

 

LOUT

 

VOUT

0.12

 

 

 

 

40μH

 

 

CS

RS

 

 

 

 

 

 

μ

C

 

1000pF

 

 

 

 

 

 

 

1k

 

 

 

 

15

RECTIFIER

1 F

OUT

 

 

 

 

 

CGG

 

MBR 3100

 

220μF

47μF

1N4148

 

 

 

1μF

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5k

 

10

11

15

 

10

7

6

 

 

 

 

DIODE

CS

VGG

OUT

SRC

 

 

 

 

 

 

 

 

 

14

VCC

 

 

 

 

EAINV

2

 

50k

 

CCC

 

 

 

 

 

 

220pF

 

 

 

 

UC3578

 

 

 

R1

 

 

 

 

 

 

 

 

1μF

 

 

 

 

 

 

 

 

 

 

16

SS

 

 

 

 

 

100k

2200pF

 

 

GND GND GND GND

EAOUT

3

 

 

 

 

 

 

 

 

 

 

4

5

12

13

 

 

 

1k

 

CSS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UDG-99064

05/99

 

 

 

 

 

 

 

 

 

 

ABSOLUTE MAXIMUM RATINGS

VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +72V EAINV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to +10V EAOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to +10V SS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to +10V DIODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to VCC VGG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to VCC +14V CS . . . . . . . . . . . . . . . . . . . . . . . . . . . . VCC – 5V to VCC +0.6V

IOUT Pulsed . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.8A to +0.6A SRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.6V to VCC

Storage Temperature . . . . . . . . . . . . . . . . . . . -65°C to +150°C Junction Temperature. . . . . . . . . . . . . . . . . . . –55°C to +150°C Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300°C

Currents are positive into, negative out of the specified terminal. Consult Packaging Section of Databook for thermal limitations and considerations of packages.

UC2578

UC3578

CONNECTION DIAGRAM

DIL-16, SOIC-16 (Top View)

N or DP Packages

Note: The four GND pins are internally connected.

ELECTRICAL CHARACTERISTICS: Unless otherwise specified VCC = 14V, VGG = 14V, TA = TJ.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNITS

Oscillator Section

 

 

 

 

 

Frequency

VCC = 14V to 72V, EAINV = 1.9V, TJ = 25°C

100

110

120

kHz

 

VCC = 11V to 14V, Over Temperature

90

 

120

kHz

Error Amplifier Section

 

 

 

 

 

EAINV

EAOUT = EAINV

1.97

2

2.03

V

IEAINV

EAOUT = EAINV

 

100

300

nA

EAVOL

EAOUT/EAINV, 25°C

70

80

 

dB

EAOUT High

EAINV £ 1.9V, IEAOUT = –100mA

5.5

6.2

 

V

EAOUT Low

EAINV ³ 2.1V, IEAOUT = 100mA

 

0.8

1.1

V

Unity Gain Bandwidth

TJ = 25°C, F = 100kHz

0.85

1

 

MHz

PSRR, EAOUT

EAOUT = EAINV, VCC = 14V

80

90

 

dB

Current Sense Comparator Section

 

 

 

 

 

Threshold (Referred to VCC)

 

0.4

0.5

0.6

V

Input Bias Current

CS = VCC – 0.4V

 

0.2

1

mA

Propagation Delay

VOVERDRIVE = 250mV

 

0.7

1.2

ms

Blanking Time

VOVERDRIVE = 250mV

75

200

300

ns

Gate Drive Output Section

 

 

 

 

 

VOH

IOUT = –200mA

9.5

11

 

V

VOL

IOUT = 20mA

 

0.2

0.36

V

 

IOUT = 200mA

 

1.5

2

V

Rise Time

TJ = 25°C, C LOAD = 1nF

 

40

70

ns

Fall Time

TJ = 25°C, C LOAD = 1nF

 

40

70

ns

Pulse Width Modulator Section

 

 

 

 

 

Maximum Duty Cycle

EAINV £ 1.9V

85

90

 

%

Minimum Duty Cycle

EAINV ³ 2.1V

 

0

 

%

Modulator Gain

EAOUT = 2.5V to 3.5V

 

30

 

%/V

Undervoltage Lockout Section

 

 

 

 

 

Start Threshold

OUT – SRC, EAINV £ 1.9V, SRC = 0V

10

11

12

V

UVLO Hysteresis

 

1.5

2

2.5

V

2

UC2578

UC3578

ELECTRICAL CHARACTERISTICS: Unless otherwise specified VCC = 14V, VGG = 14V, TA = TJ.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNITS

VGG Regulator Section

 

 

 

 

 

VGG – SRC

VCC = 72V, SRC = 0V, IVGG = –7mA

14.5

15.25

17

V

 

VCC = 50V, SRC = 0V, IVGG = –7mA

14

14.75

16

V

 

VCC = 15V, SRC = 0V, IVGG = –7mA

13

13.75

14.5

V

 

VCC = 11V, SRC = 0V, IVGG = –7mA

9.5

10

10.5

V

Soft Start Ramp Section

 

 

 

 

 

Soft Start Ramp Current

 

–30

–45

 

mA

Supply Current Section

 

 

 

 

 

IVCC

EAINV ³ 2.1V, SRC = 0V

 

10

14

mA

IVGG

EAINV ³ 2.1V, SRC = 0V

 

7

10.5

mA

PIN DESCRIPTIONS

CS: Peak current limit sense pin. Senses the current across a current sense resistor placed between VCC and the drain of the NMOS buck switch. OUT will be held low (NMOS buck switch off) if VCC – CS exceeds 0.5V.

DIODE: An external small signal diode (1N4148 typical) is connected here, anode to VCC and cathode to DIODE, to implement the VGG regulator function.

EAINV: Inverting input to error amplifier. VOUT sense feedback is connected to this pin. The non-inverting input of the error amplifier is internally connected to 2V.

EAOUT: Output of the error amplifier. Use EAOUT and EAINV for loop compensation components.

GND: Circuit Ground. The four ground pins are internally connected together by the fused leadframe of the package. They provide the primary thermal conduction path for dissipating junction heat.

OUT: Gate drive for the external NMOS switch connected between VCC and the buck inductor.

SRC: This pin is connected to the junction of the external NMOS switch source, the floating voltage source capacitor, the free-wheeling diode cathode, and buck inductor.

SS: The external soft start capacitor is connected to this pin.

VGG: An external capacitor connected from VGG to SRC completes the floating voltage source for the floating gate driver. A 1μF capacitor is recommended.

VCC: Input supply voltage. This pin supplies an internal ground referenced voltage regulator that supplies the IC and an on-chip regulated floating voltage source (VGG – SRC) used by the floating driver to drive the external NMOS buck switch. This pin should be bypassed with a high quality ceramic capacitor.

APPLICATION INFORMATION

The UC3578 Floating Buck Controller is a high frequency switching regulator with a floating driver which provides PWM control for non-isolated buck converters. The controller operates at a fixed 100 kHz switching frequency, and in voltage mode control. The duty cycle range of the PWM output is 0% to 90% allowing for a wide range of input voltages (14V minimum with transients to 72V). The regulator features an undervoltage lockout threshold of 11V with approximately 2V hysterisis as well as soft start capability. The typical application circuit shown is for a 15V to 40V input and a 12V at 3A output.

To ensure proper operation of the floating driver, an external capacitor (1μF ceramic) must be connected from VGG to SRC, and to the source of the external MOSFET

through a small resistor, as shown in the typical application diagram and in Fig 2. This capacitor provides the energy for the high side driver. The gate drive voltage to the MOSFET is internally regulated to 14V. A diode (1N4148) is required from the input voltage to DIODE. This allows the floating drive capacitor to charge during conduction of the output rectifier but prevents its discharge back into the supply rail. A 1μF ceramic capacitor is recommended from VCC to ground to provide high frequency decoupling. Additional decoupling of this pin could be accomplished by a low value resistor between VCC and VIN and a 1μF capacitor from VCC to GND as shown in the schematic.

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