LTC3814-5_15 Datasheet, PDF (24/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
about 3mA and driver current can be calculated by:
I_GATE = f(Q_G(TOP) + Q_G(BOT)), where Q_G(TOP) and Q_G(BOT)
are the gate charges of the top and bottom MOSFETs.
This loss is proportional to the supply voltage that
INTV_CC is derived from, i.e., V_IN, V_OUT or an external
supply connected to INTV_CC.
4. C_OUT loss. The output capacitor has the difficult job
of filtering the large RMS input current out of the synchro-
nous MOSFET. It must have a very low ESR to minimize
the AC I²R loss.
Other losses, including C_IN ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss. When
making adjustments to improve efficiency, the input cur-
rent is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
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
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 dis-
charge 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 problem.
Design Example
As a design example, take a supply with the following speci-
fications: V_IN = 12V ±20%, V_OUT = 24V ±5%, I_OUT(MAX) =
5A, f = 250kHz. Since V_IN can vary around the 12V nominal
value, connect a resistive divider from V_IN to V_OFF to keep
the frequency independent of V_IN changes:
R1 ₌ 12V _{− 1= 6.74}
R2 1.55V
Choose R1 = 133k and R2 = 20k. Now calculate timing
resistor R_OFF:
R_OFF
=
1+ 133k / 20k
250kHz • 76pF
=
402.6k
The duty cycle is:
D = 1− ^12V = 0.5
24V
and the maximum input current is:
I_IN(MAX
)
=
5A
1− 0.5
=
10A
Choose the inductor for about 40% ripple current at the
maximum V_IN:
L
=
12V
250kHz • 0.4
•
10A


1

12V
24V


=
6μH
The peak inductor current is:
I_L(PEAK
)
=
5A
1− 0.5
+
1(4A)
2
=
12A
so, choose the CDEP147 5.9μH inductor with I_SAT = 16.4A
at 100°C.
Next, choose the bottom MOSFET switch. Since the drain
of the MOSFET will see the full output voltage plus any
ringing, choose a 40V MOSFET to provide a margin of
safety. The Si7848DP has:
BV_DSS = 40V
R_DS(ON) = 9mΩ(max)/7.5mΩ(nom),
δ = 0.006/°C,
C_MILLER = (14nC – 6nC)/20V = 400pF,
V_GS(MILLER) = 3.5V,
θ_JA= 20°C/W.
This yields a nominal sense voltage of:
V_SNS(NOM)
=
1.7
•
0.0075Ω
1− 0.5
•
5A
=
128mV
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