LTC3734_15 Datasheet, PDF (12/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
APPLICATIONS INFORMATION
The basic LTC3734 application circuit is shown in
Figure 1 on the first page of this data sheet. External com-
ponent selection begins with the selection of the inductors
based on ripple current requirements and continues with
the current sensing resistors using the calculated peak
inductor current and/or maximum current limit. Next, the
power MOSFET, D1 is selected. The operating frequency
and the inductor are chosen based mainly on the amount
of ripple current. Finally, C_IN is selected for its ability to
handle the input ripple current and C_OUT is chosen with
low enough ESR to meet the output ripple voltage and
load step specifications. The circuit shown in Figure 1
can be configured for operation up to an input voltage of
28V (limited by the external MOSFETs).
R_SENSE Selection For Output Current
R_SENSE is chosen based on the required peak output cur-
rent. The LTC3734 current comparator has a maximum
threshold of 72mV/R_SENSE and an input common mode
range of SGND to SV_CC. The current comparator threshold
sets the peak inductor current, yielding a maximum aver-
age output current I_MAX equal to the peak value less half
the peak-to-peak ripple current, ∆I_L.
Allowing a margin for variations in the LTC3734 and external
component values yields:
R_SENSE = (40mV/I_MAX)
Operating Frequency
The LTC3734 uses a constant frequency architecture with
the frequency determined by an internal capacitor. This
capacitor is charged by a fixed current plus an additional
current which is proportional to the DC voltage applied
to the FREQSET pin. The FREQSET voltage is internally
set to 1.2V. It is recommended that this pin is actively
biased with a resistor divider to prevent noise getting
into the system.
A graph for the voltage applied to the FREQSET pin vs fre-
quency is given in Figure 2. As the operating frequency is
increased the gate drive and switching losses will be higher,
reducing efficiency (see Efficiency Considerations). The
maximum switching frequency is approximately 550kHz.
600
550
500
450
400
350
300
250
200
150
100
0
0.5 1.0 1.5 2.0 2.5 3.0
FREQSET PIN VOLTAGE (V)
3734 F02
Figure 2. Operating Frequency vs V_FREQSET
Inductor Value Calculation and Output Ripple Current
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because
MOSFET gate charge and transition losses increase
directly with frequency. In addition to this basic tradeoff,
the effect of inductor value on ripple current and low cur-
rent operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆I_L, decreases with higher
inductance or frequency and increases with higher V_IN:
∆I_L
=
V_OUT
fL


1−
V_OUT
V_IN


where f is the individual output stage operating frequency.
Accepting larger values of ∆I_L allows the use of low in-
ductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
∆I_L = 0.4(I_OUT), where I_OUT is the maximum load current.
Remember, the maximum ∆I_L occurs at the maximum
input voltage. The inductor ripple current is determined
by the frequency, inductance, input and output voltages.
3734fa
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