LTC3734 Datasheet, PDF (12/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
APPLICATIO S I FOR ATIO
The basic LTC3734 application circuit is shown in
Figure 1 on the first page of this data sheet. External
component selection begins with the selection of the
inductors based on ripple current requirements and con-
tinues with the current sensing resistors using the calcu-
lated 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
current. The LTC3734 current comparator has a maxi-
mum 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 maxi-
mum average output current I_MAX equal to the peak value
less half the peak-to-peak ripple current, ΔI_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
frequency 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.
12
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
current operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔI_L, decreases with higher
inductance or frequency and increases with higher V_IN:
ΔI_L
=
V_OUT
fL
⎛
⎝⎜^1−
V_OUT
V_IN
⎞
⎠⎟
where f is the individual output stage operating frequency.
Accepting larger values of ΔI_L allows the use of low
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
ΔI_L = 0.4(I_OUT), where I_OUT is the maximum load current.
Remember, the maximum ΔI_L occurs at the maximum
input voltage. The inductor ripple current is determined by
the frequency, inductance, input and output voltages.
3734f