LTC3418 Datasheet, PDF (13/20 Pages) Linear Technology – 8A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3418
APPLICATIO S I FOR ATIO
2. I²R losses are calculated from the resistances of the
internal switches, R_SW, and external inductor R_L. In con-
tinuous mode the average output current flowing through
inductor L is “chopped” between the main switch and the
synchronous switch. Thus, the series resistance looking
into the SW pin is a function of both top and bottom
MOSFET R_DS(ON) and the duty cycle (DC) as follows:
R_SW = (R_DS(ON)TOP)(DC) + (R_DS(ON)BOT)(1 – DC)
The R_DS(ON) for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
curves. Thus, to obtain I²R losses, simply add R_SW to R_L
and multiply the result by the square of the average output
current.
Other losses including C_IN and C_OUT ESR dissipative
losses and inductor core losses generally account for less
than 2% of the total loss.
Thermal Considerations
In most applications, the LTC3418 does not dissipate
much heat due to its high efficiency.
But, in applications where the LTC3418 is running at high
ambient temperature with low supply voltage and high
duty cycles, such as in dropout, the heat dissipated may
exceed the maximum junction temperature of the part. If
the junction temperature reaches approximately 150°C,
both power switches will be turned off and the SW node
will become high impedance.
To avoid the LTC3418 from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the
maximum junction temperature of the part. The tempera-
ture rise is given by:
T_R = (P_D)(θ_JA)
where P_D is the power dissipated by the regulator and θ_JA
is the thermal resistance from the junction of the die to the
ambient temperature. For the 38-Lead 5mm × 7mm QFN
package, the θ_JA is 34°C/W.
The junction temperature, T_J, is given by:
T_J = T_A + T_R
where T_A is the ambient temperature.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (R_DS(ON)).
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current.
When a load step occurs, V_OUT immediately shifts by an
amount equal to ∆I_LOAD(ESR), where ESR is the effective
series resistance of C_OUT. ∆I_LOAD also begins to charge or
discharge C_OUT generating a feedback error signal used by
the regulator to return V_OUT to its steady-state value.
During this recovery time, V_OUT can be monitored for
overshoot or ringing that would indicate a stability prob-
lem. The I_TH pin external components and output capaci-
tor shown in the Typical Application on the front page of
this data sheet will provide adequate compensation for
most applications.
Design Example
As a design example, consider using the LTC3418 in an
application with the following specifications: V_IN = 3.3V,
V_OUT = 2.5V, I_OUT(MAX) = 8A, I_OUT(MIN) = 200mA, f = 1MHz.
Because efficiency is important at both high and low load
current, Burst Mode operation will be utilized.
First, calculate the timing resistor:
R_OSC
=
7.3 • 10¹⁰
1• 10⁶
–
2.5k
=
70.5k
Use a standard value of 69.8k. Next, calculate the inductor
value for about 40% ripple current:
L
=
⎛
⎝⎜
2.5V ^⎞
(^1MHz)(^3.2A)⎠⎟
⎛⎝⎜_1–
2.5V
3.3V
⎞⎠⎟
=
0.19µH
3418f
13