LTC3122_15 Datasheet, PDF (11/26 Pages) Linear Technology – 15V, 2.5A Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3122
Applications Information
Overvoltage Lockout
An overvoltage condition occurs when V_OUT exceeds ap-
proximately 16.2V. Switching is disabled and the internal
soft-start ramp is reset. Once V_OUT drops below approxi-
mately 15.6V, a soft-start cycle is initiated and switching
is enabled. If the boost converter output is lightly loaded
so that the time constant product of the output capaci-
tance, C_OUT, and the output load resistance, R_OUT is near
or greater than the soft-start time of approximately 10ms,
the soft-start ramp may end before or soon after switching
resumes, defeating the inrush current limiting of the closed
loop soft-start following an overvoltage event.
Short-Circuit Protection
The LTC3122 output disconnect feature allows output
short-circuit protection. To reduce power dissipation under
overload and short-circuit conditions, the peak switch
current limit is reduced to 1.6A. Once V_OUT > 1.5V, the
current limit is set to its nominal value of 3.5A.
V_IN > V_OUT Operation
The LTC3122 step-up converter will maintain voltage regu-
lation even when the input voltage is above the desired
output voltage. Note that operating in this mode will exhibit
lower efficiency and a reduced output current capability.
Refer to the Typical Performance Characteristics section
for details.
Burst Mode Operation
When the PWM/SYNC pin is held low, the boost converter
operates in Burst Mode operation to improve efficiency
at light loads and reduce standby current at no load. The
input thresholds for this pin are determined relative to V_CC
with a low being less than 10% of V_CC and a high being
greater than 90% of V_CC. The LTC3122 will operate in
fixed frequency PWM mode even if Burst Mode operation
is commanded during soft-start.
In Burst Mode operation, the LTC3122 switches asynchro-
nously. The inductor current is first charged to 600mA
by turning on the N-channel MOSFET switch. Once this
current threshold is reached, the N-channel is turned off
and the P-channel synchronous switch is turned on, de-
livering current to the output. When the inductor current
discharges to approximately zero, the cycle repeats. In
Burst Mode operation, energy is delivered to the output
until the nominal regulation value is reached, at which
point the LTC3122 transitions to sleep mode. In sleep, the
output switches are turned off and the LTC3122 consumes
only 25μA of quiescent current. When the output volt-
age droops approximately 1%, switching resumes. This
maximizes efficiency at very light loads by minimizing
switching and quiescent losses. Output voltage ripple in
Burst Mode operation is typically 1% to 2% peak-to-peak.
Additional output capacitance (10μF or greater), or the
addition of a small feed-forward capacitor (10pF to 50pF)
connected between V_OUT and FB can help further reduce
the output ripple.
The maximum output current (I_OUT) capability in Burst
Mode operation varies with V_IN and V_OUT, as shown in
Figure 2.
350
V_OUT = 2.2V
300
V_OUT = 5V
V_OUT = 7.5V
V_OUT = 12V
250
200
150
100
50
0
0.5
1.5
2.5
3.5
4.5
5.5
V_IN, FALLING (V)
3122 F02
Figure 2. Burst Mode Maximum Output Current vs V_IN
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