ELM611DA 2A 23V
high efficiency synchronous PWM step down DC/DC converter
There is another important zero which is produced by output capacitance and ESR when output capacitor is
with large capacitance with high ESR.
This zero is located at:
fesr = 1 / (2π × Cout × Resr)
Under this situation, a third pole is produced by compensation capacitor (C4) and compensation resistor (R3)
which are used to compensate the effect of ESR zero.
This pole is located at:
fp3 = 1 / (2π × C4 × R3)
The purpose of compensation is to stabilize transfer function of DC/DC converter.
For crossover frequency, feedback loop with unity gain is important. Low crossover frequency slows the re-
sponse of line and load regulation, whereas high crossover frequency may result in unstableness of control
sys-
. To set cross frequency to be 1/10 of switching one is the easiest way.
tem
For best solution of compensation, please follow these steps:
1) Select compensation resistor (R3) based upon the desired crossover frequency. The value of R3 can be calcu-
lated by following formula:
R3 = [ (2π × Cout × fc) / (Gea × Gcs) ] × (Vout/Vfb) < [ (2π × Cout × 0.1 × fs) / (Gea × Gcs) ] × (Vout/Vfb)
fc = desired crossover frequency, which is usually set to be 1/10 of switching frequency.
2) Select compensation capacitor (C1) based upon the desired phase margin. For typical inductor values, set fz1
to be 1/10 of switching frequency so that fz1 is able to acquire sufficient phase margin to produce compensation
zero. C1 can be calculated by following formula:
C1 > 4 / (2π × R3 × fc)
R3 = compensation resistor.
3) Select the second compensation capacitor (C4) if it is required. If ESR zero is located in half of switching fre-
quency because of output capacitor, or when the following relationship is satisfied:
1 / (2π × Cout × Resr) < fs/2
the second compensation capacitor, C4 can be calculated by following formula:
C4 = (Cout × Resr) / R3
8. External bootstrap diode
An external bootstrap diode may enhance the efficiency of DC/DC converter, applicable conditions of external
BS diode are:
Vout = 5V or 3.3V, and duty cycle is high: D = Vout/Vin > 65%
Under this situation, ELM recommends using an additional external BS diode for better solution.
External BS
diode IN4148
BS
ELM611DA
SW
◄
Cbs
0.1 to 1µF
5V or 3.3V
L
Cout
Add external bootstrap diode to enhance efficiency.
*
Rev.1.2
11 - 7
ELM-TECH [ ELM Technology Corporation ]
