ISL78227 Datasheet, PDF (35/43 Pages) Intersil Corporation – 2-Phase Boost Controller with Integrated Drivers

ISL78227
When the output voltage rises back to be above the VOUT_UV
threshold of 80%*VREF_1.6V plus 4% hysteresis, PGOOD will be
released to be pulled high after a 0.5ms delay.
Equivalently, the V_OUT undervoltage threshold is set at the same
percentage of V_OUT target voltage V_{OUT_TARGET} (set by
VREF_1.6V) since the device uses the same FB voltage to
regulate the output voltage with the same resistor divider
between V_OUT and the FB pin (refer to Equation 2 on page 25).
Therefore the V_OUT undervoltage threshold is set at 80% of
V_{OUT_TARGET}. According to Equation 2 on page 25, the V_OUT
undervoltage protection threshold can be calculated using
Equation 19.
VOUT_UV
=
0.8

1.6




1
+
RR_{-----FF----BB----21-}
(EQ. 19)
OVERCURRENT LIMITING AND FAULT PROTECTION
The ISL78227 has multiple levels of overcurrent
protection/limiting. Each phase’s peak inductor current is
protected from overcurrent conditions by limiting its peak current
and the combined total current is protected on an average basis.
Also, each phase is implemented with instantaneous
cycle-by-cycle negative current limiting (OC_NEG_TH = -48µA).
Peak Current Cycle-by-Cycle Limiting (OC1)
Each individual phase’s inductor peak current is protected with
cycle-by-cycle peak current limiting (OC1) without triggering
Hiccup or Latch-off shutdown of the IC. The controller
continuously compares the CSA output current sense signal
I_SENx (calculated by Equation 11 on page 31) to an overcurrent
limiting threshold (OC1_TH = 80µA) in every cycle. When I_SENx
reaches 80µA, the respective phase’s LGx is turned off to stop
inductor current further ramping up. In such a way, peak current
cycle-by-cycle limiting is achieved.
The equivalent cycle-by-cycle peak inductor current limiting for
OC1 can be calculated by Equation 20:
I_OC1x = 80  10^–6  _R-^R----_SS----_EE----_NT----^xx- A
(EQ. 20)
Negative Current Cycle-by-Cycle Limiting (OC_NEG)
Each individual phase’s inductor current is protected with
cycle-by-cycle negative current limiting (OC_NEG) without
triggering Hiccup or Latch-off shutdown of the IC. The controller
continuously compares the CSA output current sense signal
I_SENx (calculated by Equation 11 on page 31) to a negative
current limiting threshold (OC_NEG_TH = -48µA) in every cycle.
When I_SENx falls below -48µA, the respective phase’s UGx is
turned off to stop the inductor current further ramping down. In
such a way, negative current cycle-by-cycle limiting is achieved.
The equivalent negative inductor current limiting level can be
calculated by Equation 21:
I_OCNEGx = –48  10^–6  _R-^R----_SS----_EE----_NT----^xx- A
(EQ. 21)
Peak Overcurrent Fault (OC2_PEAK) Protection
If either of the two individual phase’s current sense signal I_SENx
(calculated by Equation 11 on page 31) reaches 105µA
(OC2_TH = 105µA), the Peak Overcurrent fault (OC2_PEAK) will
be triggered. The ISL78227 will respond with fault protection
actions to shut down the PWM switching and enters either
Hiccup or Latch-off mode depending on HIC/LATCH pin
configuration as described in “Selectable Hiccup or Latch-Off Fault
Response” on page 33 and Table 3 on page 34.
This fault protection is intended to protect the device by
shutdown (Hiccup or Latch-off) from the worst case condition
where OC1 cannot limit the inductor peak current.
This fault detection is active at the beginning of soft-start (t₅ as
shown in the Figure 58 on page 29).
Under the selection of Hiccup response for the OC2_PEAK fault,
when both phases’ peak current sense signal I_SENx no longer trip
the OC2_PEAK thresholds (105µA), the device will return to
normal switching and regulation through Hiccup soft-start.
The equivalent inductor peak current threshold for the
OC2_PEAK fault protection can be calculated by Equation 22:
I_OC2x = 105  10^–6  _R-^R----_SS----_EE----_NT----^xx- A
(EQ. 22)
Constant Current Control (CC)
A dedicated constant average Current Control (CC) loop is
implemented in the ISL78227 to control the input current to be
constant at overload conditions, which means constant input
power control under certain constant input voltage.
As shown in Figure 3 on page 7, the V_IMON represents the total
input average current and is sent to the error amplifier Gm2 input to
be compared with the internal CC reference V_{REF_CC} (1.6V). Gm2
output is driving COMP voltage through a diode D_CC. Thus, the
COMP voltage can be controlled by either Gm1 output or Gm2
output through D_CC depending on load conditions.
At normal operation without overloading, V_IMON is lower than the
V_{REF_CC} (1.6V at default). Therefore, Gm2 output is HIGH and D_CC is
reversely blocked and not forward conducting. In this case, the
COMP voltage is controlled by the voltage loop error amplifier Gm1’s
output to have the output voltage regulated.
At input average current overloading case, when V_IMON reaches
V_{REF_CC} (1.6V), Gm2 output falls and D_CC is forward conducting,
and Gm2 output overrides Gm1 output to drive COMP. In this way
the CC loop overrides the voltage loop, meaning V_IMON is controlled
to be constant and input average constant current operation is
achieved. Under certain constant input voltage, input CC makes
input power constant for the boost converter. Compared to peak
current limiting schemes, the average constant current control is
more accurate to control the average current to be constant, which
is beneficial for the user to accurately control the maximum average
power for the converter to handle.
The CC current threshold should be set lower than the OC1 peak
current threshold with margin. Generally, the OC1 peak current
threshold (per phase) is set 1.5 to 2 times higher than the CC
current threshold (here referred to per phase average current).
This matches with the physics of the power devices that normally
has higher transient peak current rating and lower average
current ratings. The OC1 provides protection against the transient
peak current. The CC controls the average current with slower
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FN8808.2
February 24, 2016