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参数资料
| 型号： | GM3255S8T |
| 厂商： | American Power Management Inc. |
| 元件分类： | DC/DC变换器 |
| 英文描述： | 1.5A 280kHz DC-DC CONVERTOR BOOST REGULATOR |
| 中文描述： | 1.5A的为280kHz的DC - DC转换器升压稳压器 |
| 文件页数： | 3/19页 |
| 文件大小： | 234K |
| 代理商： | GM3255S8T |
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.
The switch current is limited to
Where:
V
= the base - emitter voltage drop of Q1, typi-
BE(Q1)
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,
R
, results in a considerable power loss which in-
SENSE
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/s.
Figure 13.Current Limiting using a
Current Sense Resistor
VC
RSENSE
Q1
VCC
R1
VIN
C2
C1
R2
C3
Output
Ground
PGND
AGND
+
-
I
=
SWITCH(PEAK)
V
BE(Q1)
R
ESENSE
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.
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
V
pin. In normal operation V
looks similar to a
SW
square wave, and is dependent on the converter topol-
ogy.
Formulas for calculating V
in the boost and flyback
SW
topologies are given in the section "V
Voltage Limit."
SW
The voltage on V
charges capacitor C3 when the
SW
switch is off, causing the voltage at the V pin to shift
C
upwards. When the switch turns on, C3 discharges
through R3, producing a negative slope at the V
pin.
C
The negative slope provides the slope compensation.
The amount of slope compensation added by this cir-
cuit is
VSW
Figure 14. Technique for Increasing Slope
Compensation
VC
R1
C2
C1
R2
R3
VSW
C3
DI
DT
= V
(
) (1 - e
) (
)
SW
R3
R2 + R3
-(1 - D)
RCf
33 SW
f
SW
(1 - D) R A
EV
G
M
3
2
5
11
相关PDF资料 |
PDF描述 |
|---|---|
| GM3255 | 1.5A 280kHz DC-DC CONVERTOR BOOST REGULATOR |
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| GM34063S8R | 1.5A DC-TO-DC CONVERTER CONTROL CIRCUIT |
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| GM3326 | 制造商:GAMMA 制造商全称:GAMMA 功能描述:PWM,PWM/PFM SWITCHING STEP-UP & DOWN DC/DC CONVERTER CONTROLLER ICS |
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