ISL78268 Datasheet, PDF (25/33 Pages) Intersil Corporation – Integrated 2A sourcing

ISL78268
VIN
RSEN
RSET _ISEN1P
-
+
RBIAS ISEN1N
VOUT
VCC
ISLOPE
SLOPE
ISEN1
VRAMP
RG
ISL1 = n*ISLOPE
VSL
ON WHILE
UG ON
ISL
CSL
ON WHILE
LG ON
+
Vrefsl = 0.5V
-
RSLOPE
ISLOPE
ma
mb
ISEN1
ISL
mSL
mma_a1₁==Mam+_am+SLm_SL
VRAMP
VRAMP = (ISEN1+ISL)*RG
FIGURE 46. SLOPE COMPENSATION BLOCK
Light Load Efficiency Enhancement
For switching mode power supplies, the total loss is related to
both the conduction loss and the switching loss. The conduction
loss dominates at heavy load while the switching loss dominates
at light load condition. The ISL78268 has the option to be set in
cycle-by-cycle Diode Emulation mode and pulse skipping
features to enhance the light load efficiency.
IMON/DE is used to select DE (Diode Emulation) mode. When the
IMON/DE is connected to an external resistor or shorted to GND,
the DE mode is selected. Also, if IMON/DE pin is pulled up to VCC
level, the device operates in Forced PWM mode.
To achieve Diode Emulation mode, the current sense amplifier
CSA2 is used to sense the output inductor current either by DCR
sensing or an accurate current shunt resistor.
DIODE EMULATION AT LIGHT LOAD CONDITION
When DE mode is selected, if the inductor current reaches
discontinuous conduction mode (DCM) operation, the ISL78268
controller will turn off the low-side MOSFET and enter into Diode
Emulation mode.
By utilizing the cycle-by-cycle diode emulation scheme, negative
current is prevented and the efficiency is improved from the
smaller RMS current in the power stage.
While in soft-start period until the PGOOD pull-down is released,
the low-side MOSFET is forced off (in either cases of DE mode or
Forced PWM mode is selected).
PULSE SKIPPING AT DEEP LIGHT LOAD CONDITION
If the converter enters Diode Emulation mode and the load is
further reduced, COMP voltage becomes lower than the
minimum threshold and the device skips the pulses to increase
the deep light load efficiency.
Average Constant Current Control
In normal PWM operation, the PWM pulse is terminated when
the sensed peak current reaches the error amplifier control
voltage. But some applications, such as charging a battery, may
desire constant output current control instead of output voltage
control. To support such requirements, ISL78268 provides the
average constant current control loop to control the average
current up to the FB regulated output voltage.
Average Constant Current control operates in the range of
approximately 25% to 100% of targeted output voltage. This is
due to the function described in the soft-start sequence (t₆-t₇)
when the FB voltage (VFB) is below 0.4V and the device operates
at 50kHz (typ) with minimum high-side MOSFET on time.
The IMON/DE pin serves to monitor the average current that is
used for average constant current control and Average
Overcurrent Protection (AVGOCP). The Current Sense Amplifier 2
(CSA2) output current, I_SEN2, which is representing the output
current (see Figure 44 for R_SEN and R_SET positions) is sourcing
out from this pin. Equation 9 describes the relation between
output current (I_OUT) and IMON/DE pin current (I_IMON). An RC
network should be connected between the IMON/DE pin and
GND, such that the ripple current signal can be filtered out and
converted to a voltage signal to represent the averaged output
current. The time constant of the RC network should be on the
order of 10 to 100 times slower than the voltage loop bandwidth
so that the programmable current limit circuit does not interfere
with the control loop stability. The IMON/DE pin voltage V_IMON
can be calculated as Equation 10.
I_IMON
=



I_O
U
T

R_{-----S---E---N--}
R_SET
+
68

10^–6

 0.125
(EQ. 9)
V_IMON = I_IMON  R_IMON
(EQ. 10)
When the IMON/DE pin voltage is at 1.6V (typ), the average
constant output current control loop on the device limits the on
time of high-side MOSFET to keep the output current constant.
While the average constant output current control is working, the
output voltage may become lower than preset output voltage
because of the lowered duty cycle. Equation 11 shows the R_IMON
for the desired average output current.
R_IMON
=
--------------------------¹----²---^.--⁸---------------------------
I_OUT

R_{-----S---E---N--}
R_SET
+
68

10^–6
(EQ. 11)
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FN8657.3
December 12, 2014