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IR3710MTRPBF
The voltage rating of the input capacitor needs to be
greater than the maximum input voltage because of
the high frequency ringing at the phase node. The
typical percentage is 25%.
Output Capacitor Selection
Select the output capacitor involves meeting the
overshoot requirement during the load removal,
transient response when the system is demanding
requires a total ESR such that the ripple voltage at
the FB pin is 7mV.
The second purpose of the output capacitor is to
minimize the overshoot of the output voltage when
the load decreases as shown in Figure 23. By using
the law of energy before and after the load removal,
equation 15 shows the output capacitance
requirement for a load step.
the current and meeting the output ripple voltage
requirement. The output capacitor has the higher
cost in the converter and increases the overall
C OUT2 =
L ? I STEP 2
V OS 2 ? V OUT 2
(15)
system cost. The output capacitor decoupling in the
converter typically includes the low frequency
capacitor, such as Specialty Polymer Aluminum, and
mid frequency ceramic capacitors.
The first purpose of output capacitors is to provide
the different energy when the load demands the
current until the inductor current reaches the load’s
current as shown in figure 22. Equation 14 shows
the charge requirement for certain load. The
advantage of IR3710 at the load step is to reduce the
delay, Tdmax, down to logic delay (in nanosecond)
compare to fix frequency control method in
microsecond or (1-D)*Ts. If the load increases right
after the PWM signal low, the longest delay of Tdmax
will be equal to the minimum lower gate on as shown
in Electrical Specification table. IR3710 also reduces
the total inductor time, which takes to reach output
current, by increasing the switching frequency up to
2.5MHz. The result reduces the recovery time.
Figure 23. Typical Output Voltage Response
Vaveform.
Boot Capacitor Selection
The boot capacitor starts the cycle fully charged to a
voltage of V B (0). An equivalent gate drive
capacitance is calculated by consulting the high side
MOSFET data sheet and taking the ratio of total gate
charge at the V5 voltage, Q G (V5), to the V5 voltage.
Q G (V5)/V5 is the equivalent gate drive capacitance
C g which will be used in the following calculations.
The voltage of the capacitor pair C B and C g after C g
becomes charged at C B ’s expense will be V B (0)- Δ V.
Choose a sufficiently small Δ V such that V B (0)- Δ V
exceeds the maximum gate threshold voltage to turn
on the high side MOSFET. Since total charge Q T is
conserved, we can write the following equations.
V B (0) ? C B = Q T = V(t on ) ? (C B + C g ) (16)
C B = C g ? ? ? B
? 1 ? ?
Figure 22. Charge Requirement during Load Step
? V (0)
? Δ V
?
?
( 1 ? D ) + 1 ? L ? Δ I OUT 2 ? (14b)
2 ( V IN ? V OUT ) ? ?
V DROP ? ?
Q = C ? V = I OUT ? T dmax + 0.5 ? I OUT ? dt (14a)
1 ?
C OUT1 = ? Δ I OUT ? ?
Fs
The output voltage drops, V DROP, initially depending
on the characteristic of the output capacitor. V DROP is
the sum of equivalent series inductance (ESL) of
output capacitor times the rate of change output
current and ESR times the change of output current.
V ESR is usually much greater than V ESL . IR3710
Choose a boot capacitor value larger than the
calculated C B . The voltage rating of this part needs to
be larger than V B (0) plus the desired derating
voltage. The voltage between BOOT and PHASE
pins must not exceed the maximum rating of IR3710.
Its ESR and ESL needs to be low in order to allow it
to deliver the large current and di/dt’s which drive
MOSFETs most efficiently. In support of these
requirements a ceramic capacitor should be chosen.
Page 14 of 20
www.irf.com
IR Confidential
4/26/10
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