SLUSFF4 Data sheet

SLUSFF4 November 2023 BQ25756E

PRODUCTION DATA

Package Options

Mechanical Data (Package|Pins)

RRV|36
- MPQF623A

Thermal pad, mechanical data (Package|Pins)

RRV|36
- QFND780

Orderable Information

9.2.1.2.9 Converter Fast Transient Response

The device integrates all the loop compensation, thereby providing a high density solution with ease of use. For faster transient reponse in reverse operating mode, the EN_CONV_FAST_TRANSIENT bit can be set to 1. If device is not used in reverse boost mode operation, this section can be disregarded.

When the converter is operating in boost mode, the non-continuous inductor current flow to the load results in a right-half plane (RHP) zero. The RHP zero location is:

Equation 36.

R H P z = \frac{V_{I N, b o o s t}}{I_{I N, b o o s t}} \frac{1}{2 π L}

For good phase margin, the unity gain bandwidth (UGBW) of the converter should be about 1/3 of the RHPz. The boost output capacitor (C_load), and the converter transient parameters (R₁, gm₁) need to be scaled to move the location of the UGBW of the converter.

Equation 37.

1 \approx \frac{{A d i v \times g m}_{1} ({s R}_{1} C_{1} + 1)}{s C_{1}} [\frac{V_{i}}{I_{o} \times 50 m}] [\frac{1}{1 + s \frac{C_{l o a d} R_{l o a d}}{2}}]

The device adjusts Adiv, gm₁ and R₁ based on the output voltage and the EN_CONV_FAST_TRANSIENT bit setting per the table below. During some boost case scenarios, the C_load needs to be adjusted to limit the converter bandwidth.

BOOST OUTPUT VOLTAGE	Adiv	C₁	EN_CONV_FAST_TRANSIENT = 0		EN_CONV_FAST_TRANSIENT = 1
BOOST OUTPUT VOLTAGE	Adiv	C₁	gm₁	R₁	gm₁	R₁
≤8 V	1/5	75 pF	0.4 μ	600 kΩ	2 μ	1.3 MΩ
8 V to 16 V	1/10	75 pF	0.47 μ	1 MΩ	2 μ	1.8 MΩ
16 V to 32 V	1/20	75 pF	0.67 μ	2.8 MΩ	2 μ	2.8 MΩ
>32 V	1/40	75 pF	2 μ	2.8 MΩ	2 μ	2.8 MΩ

As an example, assume the device operates in reverse boost mode from a 5V supply to provide a 7V boost output voltage with load up-to 5A and 10μH inductor. The RHPz is approximately located at:

Equation 38.

R H P z = \frac{V_{I N, b o o s t}}{I_{I N, b o o s t}} \frac{1}{2 π L} = 11.4 k H z

For best stability, the UGBW of the converter should be limited to 1/3 of the RHP zero, or 3.8kHz. If EN_CONV_FAST_TRANSIENT = 1, the equation becomes:

Equation 39.

1 \approx \frac{0.2 \times 2 μ (j ω \times 1.3 M Ω \times 75 p F + 1)}{j ω \times 75 p F} [\frac{5 V}{5 A \times 50 m}] [\frac{1}{1 + j ω \frac{C_{l o a d} \times 1.4}{2}}]

Solving the above for C_load gives ≥674 μF capacitor requirement.

Conversely, if EN_CONV_FAST_TRANSIENT = 0, the UGBW equation becomes:

Equation 40.

1 \approx \frac{0.2 \times 0.4 μ (j ω \times 0.6 M Ω \times 75 p F + 1)}{j ω \times 75 p F} [\frac{5 V}{5 A \times 50 m}] [\frac{1}{1 + j ω \frac{C_{l o a d} \times 1.4}{2}}]

Solving the above for C_load gives ≥51 μF capacitor requirement. However, the minimum recommended capacitor for converter stability is 80 μF, so this minimum value should be used.