LM25115SDX View Datasheet(PDF) - National ->Texas Instruments

Part Name

Description

Manufacturer

LM25115SDX

Secondary Side Post Regulator Controller

National ->Texas Instruments 

Voltage Mode Control with Current Injection (Continued)

FIGURE 7. Voltage Sensing and Feedback

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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 amplifier is level shifted by 1.27V to

produce the internal CV reference signal. The CV signal

drives a current limit amplifier with nominal transconduc-

tance of 16mA/V. The current limit amplifier has an open

drain (sink only) output stage and its output pin, CO is

typically connected to the COMP pin. During normal opera-

tion, the voltage error amplifier controls the COMP pin volt-

age which adjusts the PWM duty cycle by varying the inter-

nal CRMIX level (Figure 4). However, when the current

sense input voltage VCL 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 comparator inhibits output pulses until the CRMIX

signal increases to a normal operating level.

A current limit fold-back feature is provided by the LM25115

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 induc-

tor 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 illustrated in Figure 6 monitors the CV

signal and compares this signal to a fixed 1V threshold. This

corresponds to a negative VCL 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 VCL input falls below this threshold.

Gate Drivers Outputs (HO & LO)

The LM25115 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 + VHS - VD where VD 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 LM25115 to mini-

13

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