ISL78200_14 Datasheet, PDF (15/22 Pages) Intersil Corporation – 2.5A Regulator with Integrated High-Side MOSFET for Synchronous Buck or Boost Buck Converter

ISL78200
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 29. BOOST CONVERTER CONTROL
Referring to Figure 29, 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 29) for a desired
hysteresis VHYS at boost input voltage
R_UPM
=
-^V----^H----^Y----^S--
3A
(EQ. 3)
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-------  VBAT
1–D
(EQ. 5)
V_OUT
=
D  V_OUTBST
=
-----^D-------
1–D

VBAT
(EQ. 6)
From Equation 5 and Equation 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)
Use Equation 4 to calculate the lower resistor R_LOW (R2 in
Figure 29) according to a desired boost enable threshold.
R_LOW
=
---^R----^U----^P-----^---⁰---^.--⁸----
VFTH – 0.8
(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.
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 29) according to a desired hysteresis
VHY at boost output voltage. Use Equation 4 to calculate the
lower resistor R_LOW (R4 in Figure 29) according to a desired
boost enable threshold at boost output.
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
(V_IN *I_IN = V_OUT * I_OUT / Efficiency). If the design is to achieve the
low input operation with full load, the inductor and MOSFET have
to be selected to have enough current ratings to handle the high
current appearing at boost input. The boost inductor current are
the same with the boost input current, which can be estimated in
Equation 8, where P_OUT is the output power, VBAT is the boost
input voltage, and EFF is the estimated efficiency of the whole
boost and buck stages.
IL_IN
=
--------^P----^O----^U----^T--------
VBAT  EFF
(EQ. 8)
Based on the same concerns of boost input current, the start-up
sequence must follow the rule that the IC is enabled after the
boost input voltage rise above a certain level. The shutdown
sequence must follow the rule that the IC is disabled first before
15
FN7641.2
December 24, 2013