ISL1801 Datasheet, PDF (20/30 Pages) Intersil Corporation – sPMIC for Micro Converter Bias and Drivers

ISL1801
Precision Amplifier
The ISL1801 includes a precision amplifier for current sensing.
This low offset and low temperature drift Op amp can be used to
accurately amplify the voltage drop across a known shunt
resistor to provide a current measurement for the MCU. Due to
the very low amplitude input signal, it is necessary to include
some capacitance as a noise filter. The typical application circuit
is shown in Figure 21. The current sense gain is set by the ratio of
R1/R2. Capacitor C1 implements a low-pass filter to reduce high
frequency noise effectively averaging the desired signal. C2 is
optional and can provide additional noise filtering if required.
.
C1
AMPO
R1
AMP- ^R2
VDDREF
C2
Rs
AMP+
R2
-
I_SENSE
+
C1
R1
PGND
FIGURE 21. PRECISION CURRENT SENSE AMPLIFIER
Preload Operation
During morning start-up, shading events or evening shutdown, PV
modules may not provide enough output power to run the micro
converter. When this happens it may result in a repetitive voltage
dropout at the VR2 output. When the VR2 output powers the
MCU, this may cause a system reset after each dropout. The
repetitive restart operation of MCU may impact on the system
reliability.
To avoid this issue, the ISL1801 includes the preload feature to
test the power source before enabling VR2. This test is performed
by gradually applying a preload current to the output of
VR1/V_OUT1. If the preloaded VR1 output voltage drops below the
preset threshold, the preload is removed immediately. After the
VR1 output voltage recovers, the preload test is repeated. If VR1
can support the preload current and maintain its output voltage
above the preset threshold for some time the input power source
has enough power for the system operation. The preload is then
removed, and VR2 is enabled and will perform a soft-start.
Figure 22 shows the ISL1801s preload circuit. The four MOSFETs
each have a different ON resistance that can be connected to the
preload pin. This allows the ISL1801 to increase the preload
current step-by-step, as shown in Figure 23. When the preload is
enabled, Q1 turns on for about 500µs. During this time the
current applied to the VR1 output as shown by Equations::
I_VR1 = -_R----_P----_L----+--^V---_r-^V--_D--^R--_S--¹--_--_O----_N----_---₁-
(EQ. 8)
Next, Q2 turns on with Q1 and the preload current becomes:
I_VR1 = -_R----_P----_L----+-----_---_r--_D----_S----_--^V_O----^V-_N---^R-_--₁-¹---_--_--_r---_D----_S----_--_O----_N----_---₂---_
(EQ. 9)
Next, Q3 turns on with Q1 and Q2; Finally all the MOSFETs are on
resulting in the minimum resistance from the PRELOAD pin to
GND:
I_VR1 = _R-----_P----_L----+-----_---_r--_D----_S----_--_O-----_N----_--₁----_--_--_r---_D----_S----_--^V_O----^VN----^R_---₂-¹---_-_---_r--_D----_S----_--_O-----_N----_--₃-----_-_--_r---_D----_S----_--_O----_N-----_--₄---_
(EQ. 10)
After all the MOSFET switches are on the TIMER pin is pulled to GND
for 1ms and then released to ramp-up. If the VR1 output voltage
stays above the preset threshold until the TIMER pin ramps to 90%
of VCC5V, the preload is removed step-by-step, as shown in Figure
23. If the VR1 output drops below the preset threshold all the
switches are turned off immediately. The “Power-Up Sequence” on
page 22 provides additional details.
ISL1801
r_DS(ON)1
350
r_DS(ON)2
120
PRELOAD
r_DS(ON)3
50
r_DS(ON)4
12
VR1
OUTPUT
R_PL
Q₁
Q₂
Q₃
Q₄
FIGURE 22. PRELOAD SWITCHES
ENABLE
Q1 _OFF
ON
OFF
Q2 OFF
ON
OFF
Q3 OFF
ON
OFF
Q4 ^OFF
ON
OFF
t0 t1 t2 t3
t4 t5 t6 t7
FIGURE 23. PRELOAD TIMING DIAGRAM
WatchDog Timer
The Watchdog Timer circuit in Figure 24 verifies correct MCU
operation by monitoring the WDI input pin. The MCU must
periodically toggle the WDI pin to prevent a timeout. If a timeout
occurs, the PGOOD2 pin will be pulsed low for 0.5ms providing a
signal that can reset the MCU. PGOOD2 will then remain high
until the next timeout or invalid voltage event.
The timeout interval is determined by the capacitor and resistor
connected to the TIMER pin. Any low-to-high or high-to-low
transition on the WDI pin forces the TIMER pin low for 1.5ms to
discharge the capacitor from the TIMER pin to ground. After the
1.5ms period the TIMER pin is released so the resistor can start
charging the capacitor. When the TIMER pin voltage reaches 90%
of the VCC5V supply voltage a time-out event is triggered.
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FN8259.1
July 24, 2014