ISL78201 Datasheet, PDF (16/23 Pages) Intersil Corporation – 40V 2.5A Regulator with Integrated High-side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78201
BATTERY
+
+
VOUT_BST
R1
EXT_BOOST
R2
R3
AUXVCC
R4
0.8V
I_HYS = 3µA
0.8V
I_HYS = 3µA
LOGIC
PWM
LGATE
DRIVE
LGATE
FIGURE 32. BOOST CONVERTER CONTROL
EXT_BOOST pin is used to set boost mode and monitor the boost
input voltage. At IC start-up before soft-start, the controller will
latch in boost mode when the voltage on this pin is above
200mV; it will latch in synchronous buck mode if voltage on this
pin is below 200mV. In boost mode, the low-side driver output
PWM has the same PWM signal with the buck regulator.
In boost mode, the EXT_BOOST pin is used to monitor boost input
voltage to turn on and turn off the boost PWM. The AUXVCC pin is
used to monitor the boost output voltage to turn on and turn off
the boost PWM.
Referring to Figure 32, a resistor divider from boost input voltage
to the EXT_BOOST pin is used to detect the boost input voltage.
When the voltage on the EXT_BOOST pin is below 0.8V, the boost
PWM is enabled with a fixed 500µs soft-start when the boost
duty cycle increases from t_MINON*Fs to ~50% and a 3µA sinking
current is enabled at the EXT_BOOST pin for hysteresis purposes.
When the voltage on the EXT_BOOST pin recovers to above 0.8V,
the boost PWM is disabled immediately. Use Equation 3 to
calculate the upper resistor R_UP (R1 in Figure 32) for a desired
hysteresis VHYS at boost input voltage.
R_UPM
=
-^V----^H----^Y----^S--
3A
(EQ. 3)
Use Equation 4 to calculate the lower resistor R_LOW (R2 in
Figure 32) according to a desired boost enable threshold.
R_LOW = _V---^R-_F---^U-_T---^P_H-----^_–---⁰--₀-^.--^8.--₈--
(EQ. 4)
where VFTH is the desired falling threshold on boost input
voltage to turn on the boost, 3µA is the hysteresis current, and
0.8V is the reference voltage to be compared.
Note the boost start-up threshold has to be selected in a way that
the buck is operating well at close loop before boost start-up.
Otherwise, large inrush current at boost start-up could occur at
boost input due to the buck loop saturation. The boost startup
input voltage threshold should be set high enough to cover the
DC voltage drop of boost inductor and diode, also the buck’s
maximum duty cycle and voltage conduction drop. This ensures
buck is not reaching maximum duty cycle before boost startup.
Similarly, a resistor divider from boost output voltage to the
AUXVCC pin is used to detect the boost output voltage. When the
voltage on AUXVCC pin is below 0.8V, the boost PWM is enabled
with a fixed 500µs soft-start, and a 3µA sinking current is
enabled at AUXVCC pin for hysteresis purpose. When the voltage
on the AUXVCC pin recovers to above 0.8V, the boost PWM is
disabled immediately. Use Equation 3 to calculate the upper
resistor R_UP (R3 in Figure 32) according to a desired hysteresis
VHY at boost output voltage. Use Equation 4 to calculate the
lower resistor R_LOW (R4 in Figure 32) according to a desired
boost enable threshold at boost output.
Assuming VBAT is the boost input voltage, V_OUTBST is the boost
output voltage and V_OUT is the buck output voltage, the steady
state transfer functions are:
V_OUTBST = ₁-----¹-–----_D--  VBAT
(EQ. 5)
V_OUT = D  V_OUTBST = ₁-----^D-–----_D--  VBAT
(EQ. 6)
From Equations 5 and 6, Equation 7 can be derived to estimate
the steady state boost output voltage as a function of VBAT and
VOUT:
V_OUTBST = VBAT + VOUT
(EQ. 7)
After the IC starts up, the boost buck converters can keep
working when the battery voltage drops extremely low because
the IC’s bias (VCC) LDO is powered by the boost output. For an
example of 3.3V output application, when the battery drops to
2V, the VIN pin voltage is powered by the boost output voltage
that is 5.2V (Equation 7), meaning the VIN pin (buck input) still
needs 5.2V to keep the IC working.
Note in the above mentioned case, the boost input current could
be high because the input voltage is very low
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FN8615.1
March 31, 2015