LTC3124_15 Datasheet, PDF (12/28 Pages) Linear Technology – 15V, 5A 2-Phase Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3124
OPERATION
Selecting an R2 value of 113k to have approximately
10µA of bias current in the V_OUT resistor divider yields
the formula:
R1 = 94 • (V_OUT – 1.2V); V_OUT in Volts and R1 in kΩ.
Power converter control loop compensation is set with
a simple RC network connected between VC and SGND.
V_OUT
LTC3124
R1
FB
1.2V
R2
3124 F04
Figure 4. Programming the Output Voltage
Internal Current Limit
Current limit comparators shut off the N-channel MOSFET
switches once their respective peak current is reached.
Peak switch current per phase is limited to 3.5A, inde-
pendent of input or output voltage, unless V_OUT is below
approximately 1.5V, resulting in the current limit being
approximately half of the nominal peak values.
Lossless current sensing converts the peak current signals
of the N-channel MOSFET switches into voltages that are
summed with their respective internal slope compensation. The
summed signals are compared to the error amplifier outputs
to provide a peak current control command for the PWMs.
Zero Current Comparator
The zero current comparators monitor the inductor currents
being delivered to the output and shut off the synchronous
rectifiers when the current is approximately 50mA. This
prevents the inductor currents from reversing in polarity,
improving efficiency at light loads.
Oscillator
The internal oscillator is programmed to twice the desired
switching frequency with an external resistor from the RT
pin to SGND according to the following formula:
f_OSC
(MHz)
≅


56 
R_T (kΩ) 
=
2
•
f
(MHz)
where f = switching frequency of one phase.
Thus R_T (kΩ) ≅ 28/f (MHz). See Table 1 for various switch-
ing frequencies and their corresponding R_T values.
Table 1. Switching Frequency and Their Respective R_T
SWITCHING
FREQUENCY (kHz)
100
R_T (kΩ)
316
200
154
300
100
500
57.6
800
34.8
1000
28
1200
22.6
2000
13
2200
11.5
3000
8.06
For desired switching frequencies not included in Table 1,
please refer to the Resistance vs Frequency curve in the
Typical Performance Characteristics section.
The oscillator can be synchronized to an external frequency
by applying a pulse train of twice the desired switching
frequency to the PWM/SYNC pin. An external resistor
must be connected between RT and SGND to program the
oscillator to a frequency approximately 25% below that of
the externally applied pulse train used for synchronization.
R_T is selected in this case according to this formula:
R_T(SYNC) (kΩ) ≥ 1.25 • R_T(SWITCH) (kΩ)
where R_T(SWITCH) is the value of R_T at the desired switching
frequency, which is half of the synchronization frequency.
Shutdown
The boost converter is disabled by pulling SD below 0.25V
and enabled by pulling SD above 1.6V. Note that SD can
be driven above V_IN or V_OUT, as long as it is limited to less
than its absolute maximum rating.
Thermal Shutdown
If the die temperature exceeds 170°C typical, the LTC3124
will go into thermal shutdown (TSD). All switches will be
shut off until the die temperature drops by approximately
7°C, when the device re-initiates a soft-start and switching
is re-enabled.
3124f
12
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