LTC3444_15 Datasheet, PDF (8/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

LTC3444
U
OPERATIO
Error Amp
The LTC3444 error amplifier is a voltage mode amplifier.
The internal loop compensation is designed to optimize
transient response to control input change when the
proper output L-C and R1 values are used. Refer to
Figure 1.
Internal loop compensation is selected by grounding the
V_C pin. The loop is designed to exhibit a single pole roll-off
(–20dB/dec) with a crossover frequency of ~100KHz.
External compensation can be used by connecting the
compensation components from FB to V_C. The V_C pin
must be allowed to float when using external compensa-
tion. If external compensation is used the internal com-
pensation is automatically disabled. A Type III compensa-
tion network is typically required to meet the output
transient requirements of WCDMA.
During start-up, the ramp rate of the error amp output is
controlled to provide a soft-start function. Refer to
Figure 2.
Internal Current Limit
There are two different current limit circuits in the LTC3444.
The two circuits have internally fixed thresholds.
The first circuit sources current out of the FB pin to drop
the output voltage once the peak input current exceeds
2.5A minimum. During conditions where V_OUT is near
ground, such as during a short circuit or during startup,
this threshold is cut in half, providing current foldback
protection.
The second circuit is a high-speed peak current limit
amplifier that shuts off P-channel MOSFET switch A if the
input current exceeds 3.5A typical. The delay to output for
this amplifier is typically 50ns.
TO PWM ^VC
COMPARATORS
ERROR AMP
20μA
V_OUT
1.22V
FB
8
R1
R3
V_CONTROL
INTERNAL
COMPENSATION
NETWORK
INT
ON
R2
V_OUT
V_IN
0.5μA
GND = INTERNAL
V_{C 7} OPEN = EXTERNAL
3444 F01
Figure 1. Error Amplifier with Compensation Select Function
3444fb
8