ISL95873 查看數據表（PDF） - Intersil

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产品描述 (功能)

比赛名单

ISL95873

Buck PWM Controller with Internal Compensation and External Reference Tracking

Intersil 

ISL95873

R2

VOUT

R1

VCOMP

VDAC

FIGURE 10. NON-INTEGRATED R4TM ERROR-AMPLIFIER

CONFIGURATION

R4TM LOOP GAIN (dB)

L/C DOUBLE-POLE

p1

p2

SYSTEM HAS 2 POLES

AND 1 ZERO

CURRENT-MODE

ZERO

z1

NO COMPENSATOR IS

NEEDED

-20dB/-d2e0cdB/dec

f (Hz)

FIGURE 11. UNCOMPENSATED R4TM OPEN-LOOP RESPONSE

Transient Response

In addition to requiring a compensation zero, the integrator in

traditional architectures also slows system response to transient

conditions. The change in COMP voltage is slow in response to a

rapid change in output voltage. If the integrating capacitor is

removed, COMP moves as quickly as VOUT, and the modulator

immediately increases or decreases switching frequency to

recover the output voltage.

IOUT

VCOMP

R4TM

t

R3TM

t

VOUT

t

The dotted red and blue lines in Figure 12 represent the time

delayed behavior of VOUT and VCOMP in response to a load

transient when an integrator is used. The solid red and blue lines

illustrate the increased response of R4™ in the absence of the

integrator capacitor.

Diode Emulation

The polarity of the output inductor current is defined as positive

when conducting away from the phase node, and defined as

negative when conducting towards the phase node. The DC

component of the inductor current is positive, but the AC

component known as the ripple current, can be either positive or

negative. Should the sum of the AC and DC components of the

inductor current remain positive for the entire switching period,

the converter is in continuous-conduction-mode (CCM). However,

if the inductor current becomes negative or zero, the converter is

in discontinuous-conduction-mode (DCM).

Unlike the standard DC/DC buck regulator, the synchronous

rectifier can sink current from the output filter inductor during

DCM, reducing the light-load efficiency with unnecessary

conduction loss as the low-side MOSFET sinks the inductor

current. The ISL95873 controller avoids the DCM conduction loss

by making the low-side MOSFET emulate the current-blocking

behavior of a diode. This smart-diode operation called

diode-emulation-mode (DEM) is triggered when the negative

inductor current produces a positive voltage drop across the

rDS(ON) of the low-side MOSFET for eight consecutive PWM cycles

while the LGATE pin is high. The converter will exit DEM on the

next PWM pulse after detecting a negative voltage across the

rDS(ON) of the low-side MOSFET.

It is characteristic of the R4™ architecture for the PWM switching

frequency to decrease while in DCM, increasing efficiency by

reducing unnecessary gate-driver switching losses. The extent of

the frequency reduction is proportional to the reduction of load

current. Upon entering DEM, the PWM frequency is forced to fall

approximately 30% by forcing a similar increase of the window

voltage VW. This measure is taken to prevent oscillating between

modes at the boundary between CCM and DCM. The 30%

increase of VW is removed upon exit of DEM, forcing the PWM

switching frequency to jump back to the nominal CCM value.

Overcurrent

The overcurrent protection (OCP) setpoint is programmed with

resistor ROCSET, which is connected across the OCSET and

PHASE pins. Resistor RO is connected between the VO pin and

the actual output voltage of the converter. During normal

operation, the VO pin is a high impedance path, therefore there is

no voltage drop across RO. The value of resistor RO should always

match the value of resistor ROCSET.

FIGURE 12. R3TM vs R4TM IDEALIZED TRANSIENT RESPONSE

10

FN8390.0

December 10, 2012

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