LTC3115-2_15 Datasheet, PDF (22/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Applications Information
A significant performance advantage can be attained in
applications which have the converter output voltage pro-
grammed to 5V if the output voltage is utilized to power
the PV_CC and V_CC rails. This can be done by connecting a
Schottky diode from V_OUT to PV_CC/V_CC as shown in Figure 6.
With this bootstrap diode installed, the gate driver currents
are generated directly by the buck-boost converter at high
efficiency rather than through the internal linear regulator.
To minimize current drawn from the output, the internal
V_CC regulator contains reverse blocking circuitry which
minimizes the current into the PV_CC/V_CC pins when they
are driven above the input voltage.
The gain term, G_BUCK, is comprised of three different
components: the gain of the analog divider, the gain of the
pulse width modulator, and the gain of the power stage as
given by the following expressions where V_IN is the input
voltage to the converter, f is the switching frequency, R is
the load resistance, and t_LOW is the switch pin minimum
low time. Curves showing the switch pin minimum low time
can be found in the Typical Performance Characteristics
section of this data sheet. The parameter R_S represents the
average series resistance of the power stage and can be
approximated as twice the average power switch resistance
plus the DC resistance of the inductor.
V_OUT
PV_OUT
LTC3115-2
4.7µF
V_CC
PV_CC
31152 F06
Figure 6. Bootstrapping PV_CC and V_CC
G_BUCK = G_DIVIDERG_PWMG_POWER
G_DIVIDER
=
19.8V
V_IN
( ) G_PWM
=
3
2V
1– t_LOWf
G_POWER = _{(1– tLOW}^VI_f^N₎^R_{(R+RS )}
Buck Mode Small-Signal Model
The LTC3115-2 uses a voltage mode control loop to
maintain regulation of the output voltage. An externally
compensated error amplifier drives the VC pin to generate
the appropriate duty cycle of the power switches. Use of
an external compensation network provides the flexibility
for optimization of closed loop performance over the wide
variety of output voltages, switching frequencies, and
external component values supported by the LTC3115-2.
The small-signal transfer function of the buck-boost
converter is different in the buck and boost modes of op-
eration and care must be taken to ensure stability in both
operating regions. When stepping down from a higher
input voltage to a lower output voltage, the converter
will operate in buck mode and the small-signal transfer
function from the error amplifier output, V_C, to the con-
verter output voltage is given by the following equation:
V_O
V_C
BUCK
MODE
= G_BUCK
1+
⎛
⎝⎜
1+
s
2πf_Z
⎞
⎠⎟
s
2πf_OQ
+
⎛
⎝⎜
s
2πf_O
⎞
⎠⎟
2
Notice that the gain of the analog divider cancels the input
voltage dependence of the power stage. As a result, the
buck mode gain is well approximated by a constant as
given by the following equation:
G_BUCK
= 29.7 ^R
R + R_S
≅
29.7 = 29.5dB
The buck mode transfer function has a single zero which
is generated by the ESR of the output capacitor. The zero
frequency, f_Z, is given by the following expression where
R_C and C_O are the ESR and value of the output filter ca-
pacitor respectively.
f_Z
=
2π
1
R_CC_O
In most applications, an output capacitor with a very low
ESR is utilized in order to reduce the output voltage ripple
to acceptable levels. Such low values of capacitor ESR
result in a very high frequency zero and as a result the
zero is commonly too high in frequency to significantly
impact compensation of the feedback loop.
31152fa
22
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