ML1565 datasheet(15/21 Pages) MINILOGIC

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P15/21

Rev. C, Sep 2005

ML1565

Design Procedure (3)

Step-Up Component Selection

Step-Up compensation

The inductor and output capacitor are usually chosen first

in consideration of performance, size, and cost. The

compensation resistor and capacitor are then chosen to

optimize control-loop stability. In some cases it may help to

readjust the inductor or output capacitor value to get

optimum results. For typical designs, the component

values in the circuit of Figure 1 yield good results.

The step-up converter employs current-mode control,

thereby simplifying the control-loop compensation. When

the converter operates with continuous inductor current

(typically the case), a right-half-plane zero (RHPZ)

appears in the loop-gain frequency response. To ensure

stability, the control-loop gain should crossover (drop

below unity gain) at a frequency (fC) much less than that of

the right-half-plane zero.

The

relevant

characteristics

for

step-up

channel

compensation are:

1) Transconductance (from FBSU to COMPSU), gmEA

(135µS)

2) Current-sense amplifier transresistance, RCS, (0.3V/A)

3) Feedback regulation voltage, VFB(1.25V)

4) Step-up output voltage, VSUOUT, in V

5) Output load equivalent resistance, RLOAD,

in Ω = VSUOUT / ILOAD

The key steps for step-up compensation are:

1) Place fC sufficiently below the RHPZ and calculate Cc.

2) Select Rc based on the allowed load-step transient. Rc

sets a voltage delta on the COMP pin that corresponds

to load current step.

3) Calculate the output filter capacitor (COUT) required to

allow the Rc and Cc selected.

4) Determine if Cp is required (if calculated to be>10pF).

For

continuous

conduction,

the

right-plane

zero

frequency (fRHPZ) is given by:

fRHPZ

VOUTSU (1-D)

2 / (2π L ILOAD)

where D = the duty cycle = 1 – (VIN / VOUT), L is the inductor

value, and ILOAD is the maximum output current. Typically

target crossover (fc) for 1/6 the RHPZ. For example, if we

assume VIN = 2V, VOUT = 3.35V, and IOUT = 0.5A, then

RLOAD = 6.7Ω.

If we select L = 3.3µH then:

fRHPZ = 3.35(2/3.35)

2 / (2π x 4.7 x 10-6 x 0.5) = 115kHZ

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Choose fc = 200kHz. Calculate Cc:

Cc = (VFB/ VOUT)(RLOAD/ RCS)(gm/ 2π fc)(1-D)

=(1.25/3.35)(6.7/0.3) x (135µS/6.28 x 20kHz)

(2/3.35) = 5.35nF

Choose 6.8nF. Now select Rc such that transient droop

requirements are met. For example, if 4% transient droop

is allowed, the input to the error amplifier moves 0.04 x

1.25V, or 50mV. The error amp output drives 50mV x

135µS, or 6.75µA, across Rc to provide transient gain.

Since the current-sense transresistance is 0.3 V/A, the

value of Rc that allows the required load step swing:

Rc = 0.3 IIND(PK) / 6.75µA

In a step-up DC-to-DC converter, if LIDEAL is used, output

current relates to inductor current by:

IIND(PK) = 1.25 IOUT/ (1-D) = 1.25 IOUT VOUT / VIN

Thus, for a 400mA output load step with VIN = 2V and VOUT

= 3.35V:

Rc = [1.25(0.3 x 0.4 x 3.35) / 2]] / 6.75µA = 37kΩ

Note that the inductor does not limit the response in this

case since it can ramp at 2V/3.3µH, or 606mA/µs. The

output filter capacitor is then chosen so that the COUT RLOAD

pole cancels the Rc Cc Zero.

COUT RLOAD = Rc Cc

For example:

COUT = 37KΩ x 6.8nF / 6.7 = 37.5µF

Since a reasonable value for COUT is 47µF rather than 37.5,

choose 47µF and rescale Rc:

Rc = 47µF x 6.7 / 6.8nF = 46.3kΩ

which provides a slightly higher transient gain and

consequently less transient droop than previously selected.

If the output filter capacitor has significant ESR, a zero

occurs at:

ZESR = 1 / (2π COUT RESR)

If ZESR > fc, it can be ignored, as is typically the case with

ceramic output capacitors. If ZESR is less than fc, it should

be cancelled with a pole set by capacitor Cp connected

from COMPSU to GND:

Cp = COUT RESR / RC

If Cp is calculated to be < 10pF, it can be omitted.

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