參數(shù)資料
型號(hào)： LM5115MMX
廠商： National Semiconductor Corporation
英文描述： Secondary Side Post Regulator Controller
中文描述： 次級(jí)端后穩(wěn)壓控制器
文件頁(yè)數(shù)： 13/17頁(yè)
文件大小： 725K
代理商： LM5115MMX
Voltage Mode Control with Current Injection
(Continued)
Current Limiting (CS, CO and
VOUT)
Current limiting is implemented through the current sense
amplifier as illustrated in
Figure 6
. The current sense ampli-
fier monitors the inductor current that flows through a sense
resistor connected between CS and VOUT. The voltage gain
of the current sense amplifier is nominally equal to 16. The
output of current sense signal is shifted by 1.27V to produce
the internal CV reference signal. The CV signal drives a
current limit amplifier with nominal transconductance of
16mA/V. The current limit amplifier has an open drain (sink
only) output stage and its output pin CO is typically con-
nected to the COMP pin. During normal operation, the volt-
age error amplifier controls the COMP pin voltage which
adjusts the PWM duty cycle by varying the internal CRMIX
level (
Figure 4
). However, when the current sense input
voltage V
exceeds 45mV, the current limit amplifier pulls
down on COMP through the CO pin. Pulling COMP low
reduces the CRMIX signal below the CV signal level. When
CRMIX does not exceed the CV signal, the PWM compara-
tor inhibits output pulses until the CRMIX signal increases to
a normal operating level.
Acurrent limit fold-back feature is provided by the LM5115 to
reduce the peak output current delivered to a shorted load.
When the common mode input voltage to the current sense
amplifier (CS and VOUT pins) falls below 2V, the current limit
threshold is reduced from the normal level.At common mode
voltages
>
2V, the current limit threshold is nominally 45mV.
When VOUT is reduced to 0V the current limit threshold
drops to 36mV to reduce stress on the inductor and power
MOSFETs.
Negative Current Limit
When inductor current flows from the regulator output
through the low side MOSFET, the input to the current sense
comparator becomes negative. The intent of the negative
current comparator is to protect the low side MOSFET from
excessive currents. Negative current can lead to large nega-
tive voltage spikes on the output at turn off which can dam-
age circuitry powered by the output. The negative current
comparator threshold is sufficiently negative to allow induc-
tor current to reverse at no load or light load conditions. It is
not intended to support discontinuous conduction mode with
diode emulation by the low side MOSFET. The negative
current comparator shown illustrated in
Figure 6
monitors
the CV signal and compares this signal to a fixed 1V thresh-
old. This corresponds to a negative V
voltage between CS
and VOUT of -17mV. The negative current limit comparator
turns off the low side MOSFET for the remainder of the cycle
when the V
CL
input falls below this threshold.
Gate Drivers Outputs (HO & LO)
The LM5115 provides two gate driver outputs, the floating
high side gate driver HO and the synchronous rectifier low
side driver LO. The low side driver is powered directly by the
VCC regulator. The high side gate driver is powered from a
bootstrap capacitor connected between HB and HS. An
external diode connected between VCC and HB charges the
bootstrap capacitor when the HS is low. When the high side
MOSFET is turned on, HB rises with HS to a peak voltage
equal to VCC + V
- V
where V
is the forward drop of the
external bootstrap diode. Both output drivers have adaptive
dead-time control to avoid shoot through currents. The adap-
tive dead-time control circuit monitors the state of each
driver to ensure that the opposing MOSFET is turned off
before the other is turned on. The HB and VCC capacitors
should be placed close to the pins of the LM5115 to minimize
20134916
FIGURE 7. Voltage Sensing and Feedback
L
www.national.com
13
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