ISL78200 Datasheet, PDF (15/19 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 28. BOOST CONVERTER CONTROL
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
⋅
V
B
AT
(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)
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 startup
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
the boost input power source is turned off. At boost mode
applications where there is no external control signal to
enable/disable the IC, an external input UVLO circuit must be
implemented for the startup and shutdown sequence.
PFM is not available in boost mode.
15
FN7641.0
September 22, 2011