Datasheet

LTC3129
11
3129fa
For more information www.linear.com/LTC3129
operaTion
When stepping down from a high input voltage to a lower
output voltage, the converter operates in buck mode and
switch D remains on for the entire switching cycle except
for the minimum switch low duration (typically 90ns). Dur
-
ing the switch low duration, switch C is turned on which
for
ces SW2 low and charges the flying capacitor, C
BST2
.
This ensures that the switch D gate driver power supply
rail on BST2 is maintained. The duty cycle of switches A
and B are adjusted to maintain output voltage regulation
in buck mode.
If the input voltage is lower than the output voltage, the
converter operates in boost mode. Switch A remains on
for the entire switching cycle except for the minimum
switch low duration (typically 90ns). During the switch
low duration, switch B is turned on which forces SW1
low and charges the flying capacitor, C
BST1
. This ensures
that the switch A gate driver power supply rail on BST1 is
maintained. The duty cycle of switches C and D are adjusted
to maintain output voltage regulation in boost mode.
Oscillator
The LTC3129 operates from an internal oscillator with a
nominal fixed frequency of 1.2MHz. This allows the
DC/DC
converter efficiency
to be maximized while still using small
external components.
Current Mode Control
The LTC3129 utilizes average current mode control for the
pulse width modulator. Current mode control, both average
and the better known peak method, enjoy some benefits
compared to other control methods including: simplified
loop compensation, rapid response to load transients and
inherent line voltage rejection.
Referring to the Block Diagram, a high gain, internally
compensated transconductance amplifier monitors Vout
through a voltage divider connected to the FB pin. The
error amplifier output is used by the current mode control
loop to command the appropriate inductor current level.
The inverting input of the internally compensated average
current amplifier is connected to the inductor current
sense circuit. The average current amplifier's output is
compared to the oscillator ramps, and the comparator
outputs are used to control the duty cycle of the switch
pins on a cycle-by-cycle basis.
The voltage error amplifier monitors the output voltage,
V
OUT
through a voltage divider and makes adjustments to
the current command as necessary to maintain regulation.
The voltage error amplifier therefore controls the outer
voltage regulation loop. The average current amplifier
makes adjustments to the
inductor current
as directed by
the voltage error amplifier output via V
C
and is commonly
referred to as the inner current loop amplifier.
The average current mode control technique is similar to
peak current mode control except that the average current
amplifier, by virtue of its configuration as an integrator,
controls average current instead of the peak current. This
difference eliminates the peak to average current error
inherent to peak current mode control, while maintaining
most of the advantages inherent to peak current mode
control.
Average current mode control requires appropriate com
-
pensation for
the inner current loop, unlike peak current
mode control. The compensation network must have high
DC gain to minimize errors between the actual and com
-
manded average current level, high bandwidth to quickly
change the commanded current level following transient
load steps and a controlled mid-band gain to provide a
form of slope compensation unique to average current
mode control. The compensation components required
to ensure proper operation have been carefully selected
and are integrated within the LTC3129.
Inductor Current Sense and Maximum Output Current
As part of the current control loop required for current
mode control, the LTC3129 includes a pair of current
sensing circuits that measure the buck-boost converter
inductor current.
The
voltage error amplifier output, V
C
, is internally clamped
to a nominal level of 0.6V. Since the average inductor
current is proportional to V
C
, the 0.6V clamp level sets
the maximum average inductor current that can be pro-
grammed by
the inner current loop. Taking into account
the
current sense amplifier's gain, the maximum average