LTC3444_15 Datasheet, PDF (9/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

U
OPERATIO
Reverse Current Limit
The LTC3444 always operates in forced continuous con-
duction mode. The reverse current limit amplifier moni-
tors the inductor current from the output through switch
D. Once the negative inductor current exceeds 3A mini-
mum, the LTC3444 will shut off switch D. The high reverse
current is required to meet the transient slew require-
ments for WCDMA power amplifiers.
Output Overvoltage Protection
The LTC3444 provides output overvoltage protection. If
the output voltage exceeds 5.3V typical, P-channel MOSFET
switches A and D are turned off and N-channel MOSFET
switches B and C are turned on. Normal switching will
LTC3444
resume once the output voltage drops below ~5.1V. If the
condition which caused the output overvoltage is still
present the output will charge up to 5.3V again and the
overvoltage cycle will be repeat. Normal output regulation
will resume once the condition responsible for the output
overvoltage is removed.
Soft-Start
The soft-start function is initiated when the SHDN pin is
brought above 1.4V and the LTC3444 is out of UVLO
(above minimum input operating specs). The LTC3444 is
enabled but the PWM duty cycle is clamped via the error
amp output. The soft-start time is internally set to 250μs
to minimize output overshoot. A detailed diagram of this
function is shown in Figure 2.
V_{IN SOFT-START}
CLAMP
ERROR AMP
20μA
1.22V
FB
8
V_{C 7}
TO PWM ^VCI
COMPARATORS
I_SS
C_SS
SHDN
1
1V
3444 F02
Figure 2. Soft-Start Circuitry
3444fb
9