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PDF GM3255 Data sheet ( Hoja de datos )

Número de pieza GM3255
Descripción DC-DC CONVERTOR BOOST REGULATOR
Fabricantes Gamma 
Logotipo Gamma Logotipo



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Description
Features
GM3255 products are 280 kHz switching regulators with a
high efficiency, 1.5 A integrated switch. These parts oper-
ate over a wide input voltage range, from 2.7 V to 30 V.
The flexibility of the design allows the chips to operate in
most power supply configurations, including boost, flyback,
forward, inverting, and SEPIC. The ICs utilize current
mode architecture, which allows excellent load and line reg-
ulation, as well as a practical means for limiting current.
Combining high frequency operation with a highly inte-
grated regulator circuit results in an extremely compact
power supply solution. The circuit design includes provi-
sions for features such as frequency synchronization, shut-
down, and feedback controls for either positive or negative
voltage regulation.
Integrated Power Switch: 1.5 A Guaranteed
Input Voltage Range: 2.7 V to 30 V
High Frequency Allows for Small Components
Minimum External Components
Easy External Synchronization
Built in Overcurrent Protection
Frequency Foldback Reduces Component
Stress During an Overcurrent Condition
Thermal Shutdown with Hysteresis
Regulates Either Positive or Negative Output
Voltages
Shut Down Current: 50 µA Maximum
Wide Temperature Range Commercial
Commercial Grade : 0 to 70°C (GM3255)
Application
Boost Regulators
CCFL Backlight Driver
Laptop Computer Supplies
TYPICAL APPLICATION CIRCUITS
Multiple Output Flyback Supplies
Inverting Supplies
TFT LCD Bias Supplies
R2
3.72K
C1
0.01µF
SS
GM3255
1 VC
VSW 8
2 FB PGND 7
3 Test AGND 6
4 SS
VCC 5
3.3V
R1
5k
R3
1.28k
+C2
22µF
D1
MBRS120T3
VOUT
5V
L1
22µH
+C3
22µF
www.gammamicro.com 1

1 page




GM3255 pdf
ELECTRICAL CHARACTERISTICS
(2.7 V < VCC < 30 V; Industrial Grade: 0°C < TJ < 70°C; Commercial Grade: 0°C < TJ < 125°C; unless otherwise noted)
CHARACTERISTICS
Power Switch Section
Switch Saturation Voltage
Switch Current Limit
Minimum Pulse Width
D ICC / DVSW
Switch Leakage
General Section
Operating Current
Shutdown Mode Current
Minimum Operation Input
Voltage
Thermal Shutdown
Thermal Hysteresis
TEST CONDITIONS
MIN
ISWITCH = 1.5A, (Note 2)
ISWITCH = 1.0A, 0°C TJ 85°C
ISWITCH=1.0A, -40°C TA 0°C(Note 2)
ISWITCH = 10mA
50% duty cycle(Note 2)
80% duty cycle(Note 2)
FB = 0 V, ISW =4.0A(Note 2 )
2.7 V VCC 12V, 10mA ISW 1.0A
12V VCC 30V, 10mA ISW 1.0A
2.7 V VCC 12V, 10mA ISW 1.5A
(Note 2)
12V VCC 30V, 10mA ISW 1.5A
(Note 2)
VSW = 40 V, VCC = 0V
-
-
-
-
1.6
1.5
200
-
-
-
-
-
ISW = 0
VC < 0.8V, SS = 0V, 2.7V VCC 12V
VC < 0.8V, SS = 0V, 12V VCC 30V
VSW switching, maximum ISW = 10mA
(Note2)
(Note2)
-
-
-
-
150
-
(Note 2) Guaranteed by design, not 100% tested in production.
TYP MAX UNIT
0.8
0.55
0.75
0.09
1.9
1.7
250
10
-
17
-
2.0
1.4
1.00
1.30
0.45
2.4
2.2
300
30
100
30
V
A
ns
mA / A
100
100 µA
5.5
12
-
2.45
180
25
8.0
60
100
2.70
210
-
mA
µA
V
°C
°C
5

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GM3255 arduino
Another solution to the current limiting problem is to
externally measure the current through the switch us-
ing a sense resistor. Such a circuit is illustrated in
Figure 13.
VCC
VIN
+
-
PGND AGND VC
R2
Q1
C3
RSENSE
R1
C1
C2
Figure 13.Current Limiting using a
Current Sense Resistor
Output
Ground
The switch current is limited to
ISWITCH(PEAK)
=
VBE(Q1)
RESENSE
Where:
V BE(Q1) = the base - emitter voltage drop of Q1, typi-
cally 0.65V.
The improved circuit does not require a require a
regulated voltage to operate properly. Unfortunately, a
price must be paid for this convenience in the overall
efficiency of the circuit. The designer should note that
the input and output grounds are no longer common.
Also, the addition of the current sense resistor,
RSENSE, results in a considerable power loss which in-
crease with the duty cycle. Resistor R2 and capacitor
C3 form a low - pass filter to remove noise.
Subharmonic Oscillation
Subharmonic oscillation (SHM) is a problem found
in Current-mode control systems, where instability re-
sults when duty cycle exceeds 50%. SHM only occurs
in switching regulators with a continuous inductor cur-
rent. This instability is not harmful to the converter
and usually does not affect the output voltage regula-
tion. SHM will increase the radiated EM noise from
the converter and can cause, under certain circum-
stances, the inductor to emit high - frequency audile
noise.
SHM is an easily remedied problem. The rising slope
of the inductor current is supplemented with internal
“slope compensation” to prevent any duty cycle insta-
bility from carrying through to the next switching cycle.
In the GM3255, slope compensation is added during
the entire switch on-time, typically in the amount of
180 mA/µs.
In some cases, SHM can rear its ugly head despite
the presence of the onboard slope compensation. The
simple cure to this problem is more slope compensa-
tion avoid the unwanted oscillation. In that case, an ex-
ternal circuit, shown in Figure 14, can be added to in-
crease the amount of slope compensation used. This
circuit requires only a few components and is "tacked
on" to the compensation network.
VSW
VSW
VC
R1
C1
C2
R2
C3 R3
Figure 14. Technique for Increasing Slope
Compensation
The dashed box contains the normal compensation
circuitry to limit the bandwidth of the error amplifier.
Resistors R2 and R3 form a voltage divider off of the
VSW pin. In normal operation VSW looks similar to a
square wave, and is dependent on the converter topol-
ogy.
Formulas for calculating VSW in the boost and flyback
topologies are given in the section "VSW Voltage Limit."
The voltage on VSW charges capacitor C3 when the
switch is off, causing the voltage at the VC pin to shift
upwards. When the switch turns on, C3 discharges
through R3, producing a negative slope at the VC pin.
The negative slope provides the slope compensation.
The amount of slope compensation added by this cir-
cuit is
DI
DT
=
R3
VSW (R2 + R3 )
(1
-
e
-(1 - D)
R3C3fSW
)
( (1
-
fSW
D) REAV
)
11

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