SY21219

SilergyBuck converterSpecs verifiedDraft notes

4.5–30 V input, 6 A synchronous buck with 500 kHz constant on-time control and a power-good output, in a QFN2×3-14 package.

Overview

A high-current buck for 12 V, 19 V and 24 V rails, with output set by VOUT = 0.6 × (1 + RH/RL). Silergy's constant on-time control reacts almost instantly to load steps and stays stable with low-ESR ceramic output capacitors without any external compensation network, which is why the reference design gets away with only a handful of passives around it.

4.5–30 VInput voltage

0.6–24 VOutput voltage

6 AOutput current

500 kHzSwitching frequency

QFN2×3-14Package

Specifications verified. Every value in the specifications was checked against the manufacturer's datasheet (Rev 1.0 (June 2025)).

Features

• 4.5–30 V input voltage range, up to 6 A output current
• Adjustable output from 0.6 V to 24 V
• Low-resistance internal switches: 40 mΩ top, 20 mΩ bottom
• 500 kHz pseudo-constant switching frequency
• Constant on-time, ripple-based control for fast transient response
• ±1.5% 0.6 V reference over -40 to 125 °C
• Precise EN threshold and internal 1 ms soft-start
• Smooth start-up with a pre-biased output
• PFM light-load operation for high efficiency
• Power-good indicator and output auto-discharge
• Cycle-by-cycle valley and peak current limits, hiccup-mode output under-voltage protection, auto-recovery over-temperature protection and input UVLO
• Compact QFN2×3-14 package, MSL 1

Specifications

Key specifications

Category	Buck converter
Topology	Synchronous buck, constant on-time, ripple-based control
Input voltage	4.5–30 V
Output voltage	0.6–24 V
Maximum output current	6 A
Switching frequency	500 kHz
Quiescent current	120 µA typical
Package	QFN2×3-14

Input, output and reference

Reference voltage	0.6 V, 0.591–0.609 V over -40 to 125 °C
Shutdown current	5 µA typical, 10 µA max
Soft-start time	1 ms typical (0.7–1.3 ms)
Input UVLO	4.4 V rising max, 0.3 V hysteresis
EN thresholds	1.2 V rising, 1.0 V falling (typical)
Internal VCC	3.3 V LDO, needs 2.2 µF
Output auto-discharge	80 mA typical at VOUT = 5 V

Switching and power stage

Switching frequency range	425–575 kHz (500 kHz typical)
Internal switch RDS(on)	40 mΩ top / 20 mΩ bottom (typical)
Current limits	9 A top (peak), 6 A bottom (valley), typical
Minimum on / off time	50 ns / 100 ns
Maximum duty cycle	98%, or 100% with an external supply of more than 2.2 V across BS–LX

Protection and monitoring

Power good	Open drain; high between 90% and 120% of VREF (typical), 200 µs rise delay, 40 µs fall delay
Output short protection	Hiccup (2 ms on / 6 ms off), triggers below 50% of VREF
Over-temperature protection	150 °C, 15 °C hysteresis, auto-recovery

Package, ratings and thermal

Orderable part number	SY21219WYQ (top mark FNExyz)
Absolute max on IN	31 V
Thermal resistance θJA	22 °C/W (QFN2×3-14, 8.5×8.5 cm 4-layer board, 2 oz)
Moisture sensitivity	MSL 1

Recommended components

Feedback divider and inductor

Values from the datasheet's typical-application table.

Vout (V)	RH (kΩ)	RL (kΩ)	CFF (pF)	Inductor
1.2	100	100	220	1.0 µH, PCMB104T-1R0MT
3.3	100	22.1	220	3.3 µH, PCMB104T-3R3MS
5	100	13.7	220	4.7 µH, PCMB104T-4R7MS
12	100	5.26	220	6.8 µH, PCMB104T-6R8MS

Output capacitance and feedforward

Pick a row for your output voltage, inductor and output capacitance.

Vout (V)	Inductor (µH)	COUT range (µF)	RFF (kΩ)	CFF (pF)
1.2	0.68	66–500	1	220
1.2	1.0	66–500	1	220
1.2	1.5	66–500	1	220
3.3	2.2	66–500	1	220
3.3	3.3	66–500	1	220
3.3	4.7	66–100	1	220
3.3	4.7	100–500	1	470
5	3.3	66–500	1	220
5	4.7	66–120	1	220
5	4.7	120–500	1	470
5	5.6	66–500	1	470
12	4.7	66–120	1	220
12	4.7	120–500	1	470
12	5.6	66–120	1	220
12	5.6	120–500	1	680
12	6.8	66–220	1	470
12	6.8	220–500	1	1000

Design notes

Before you build

• EN: don’t tie it straight to VIN. The datasheet says to use a 1 kΩ to 1 MΩ resistor, and a divider on EN lets you raise the start-up voltage. Its reference design uses 10 kΩ and 1 MΩ.
• Power good is open drain. Pull it up through 10 kΩ to 100 kΩ. It’s actively held low until the output is ready.
• Required small parts: a 0.1 µF bootstrap capacitor from BS to LX, a 2.2 µF capacitor on VCC, and at least 10 µF of input capacitance (the reference design uses X5R rated 50 V, with an additional 0.1 µF for high-frequency noise).
• Output capacitance: the datasheet’s recommended-values table covers 66 to 500 µF. It suggests a 1 kΩ + 220 pF feedforward network across the upper feedback resistor, and 2.2 nF when output capacitance exceeds 500 µF.
• Light-load behavior: it switches in PFM, so at very light or no load the switching frequency can drop into the audible range.
• The 98% duty limit is an open-loop figure. Reaching 100% needs an external supply of more than 2.2 V across BS and LX.
• Thermal: the 22 °C/W figure is measured on a large 4-layer 2 oz board. The datasheet says matching it on a small board takes wide copper and many thermal vias, and recommends a 4-layer layout.

Layout guidance

• Connect the output capacitor negatives to ground with wide copper, not vias, and take the feedback sense with a Kelvin connection at the output capacitor, away from LX and BS.

Ordering and documents

Part number	SY21219
Package	QFN2×3-14
Datasheet	Manufacturer datasheet ↗ · Rev 1.0 (June 2025)
LCSC	No LCSC listing confirmed yet
Verification	Specifications checked against the datasheet

Used in recipes

• SY21219 6 A synchronous buck: 5 V at 6 A from up to 30 V

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