ST L6738A User Manual
Features
■ Flexible power supply from 5 V to 12 V bus
■ Power conversion input as low as 1.5 V
■ Light-load efficiency optimization
■ Embedded bootstrap diode
■ VIN detector
■ 0.8 V internal reference
■ 0.5% output voltage accuracy
■ Remote GND recovery
■ High-current integrated drivers
■ Sensorless and programmable precise-OC
sense across Inductor DCR
■ OV protection
■ Programmable oscillator up to 600 kHz
■ LS-less to manage pre-bias startup
■ Adjustable output voltage
■ Disable function
■ Internal soft-start
■ VFQFPN 16 3x3 mm package
Applications
■ Memory and termination supply
■ Subsystem power supply (MCH, IOCH, PCI)
■ CPU and DSP power supply
■ Distributed power supply
■ General DC-DC converter
L6738A
Single-phase PWM controller
with light-load efficiency optimization
VFQFPN16
and the device supply voltage ranging from 5 V to
12 V bus.
The L6738A features a proprietary algorithm that
allows Light-Load efficiency optimization,
boosting efficiency without compromising the
output voltage ripple.
The integrated 0.8 V reference allows generation
of output voltages with ±0.5% accuracy over line
and temperature variations.
The oscillator is programmable up to 600 kHz.
The L6738A provides a programmable
overcurrent protection and overvoltage protection.
The current information is monitored across the
inductor DCR.
The L6738A is available in a VFQFPN 16 3x3 mm
package.
Table 1. Device summary
Order code Package Packing
L6738A VFQFPN16 Tube
Description
L6738ATR VFQFPN16 Tape and reel
The L6738A is a single-phase step-down
controller with integrated high-current drivers that
provides complete control logic and protection to
realize a DC-DC converter.
The device flexibility allows to manage
conversions with power input V
January 2012 Doc ID 18134 Rev 2 1/32
as low as 1.5V
IN
www.st.com
32
Contents L6738A
Contents
1 Typical application circuit and block diagram . . . . . . . . . . . . . . . . . . . . 5
1.1 Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Pin description and connection diagrams . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 Pin descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4 Device description and operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1 LS-less startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6 Output voltage setting and protections . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1 Overcurrent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2 Overcurrent threshold setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7 Main oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8 High Current embedded drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1 Boot capacitor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.2 Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9 Application details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1 Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.2 Layout guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1 Inductor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.2 Output capacitor(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2/32 Doc ID 18134 Rev 2
L6738A Contents
10.3 Input capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
11 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
12 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Doc ID 18134 Rev 2 3/32
List of tables L6738A
List of tables
Table 1. Device summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Table 2. Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Table 3. Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table 4. Absolute maximum ratings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 5. Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 6. L6738A Protection at a glance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4/32 Doc ID 18134 Rev 2
L6738A List of figures
List of figures
Figure 1. Typical application circuit (fast protections) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 2. Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 3. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 4. Pin connection (top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 5. LS-less startup (left) vs. non-LS-less startup (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 6. Current reading network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 7. ROSC vs. switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 8. Bootstrap capacitor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 9. PWM control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Figure 10. Example of type III compensation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 11. Power connections (heavy lines) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Figure 12. Drivers turn-on and turn-off paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Figure 13. Inductor current ripple vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Figure 14. VFQFPN16 mechanical data and package dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Doc ID 18134 Rev 2 5/32
Typical application circuit and block diagram L6738A
1 Typical application circuit and block diagram
1.1 Application circuit
Figure 1. Typical application circuit (fast protections)
VCC = 5V to 12V
C
DEC
C
HF
R
VIN = 1.5V to 19V
C
BULK
L
C
Vout
C
OUT
LOAD
C
C
R
P
I
I
PGOOD
EN
C
F
R
F
GND
VSEN
VCC
VCCDR
BOOT
UGATE
PHASE
L6738A
LGATE
CSP
CSN
R
OSC
R
FB
OSC
PGOOD
EN
COMP
FB
R
OS
FBG
C
DEC
HS
LS
L6738A Reference Schematic
6/32 Doc ID 18134 Rev 2
L6738A Typical application circuit and block diagram
Figure 2. Typical application circuit
VCC = 5V to 12V
CDEC
CHF
R
VIN = 1.5V to 19V
CBULK
L
C
COUT
Vout
LOAD
C
C
R
P
I
I
PGOOD
EN
C
F
R
F
OSC
OSC
GND
VCCDR
CDECR
VCC
PGOOD
EN
BOOT
COMP
UGATE
PHASE
FB
R
R
FB
OS
FBG
L6738A
LGATE
CSP
CSN
HS
LS
VSEN
L6738A Reference Schematic
R
OS
R
FB
Doc ID 18134 Rev 2 7/32
Typical application circuit and block diagram L6738A
1.2 Block diagram
Figure 3. Block diagram
CSN
CSP
OSC
EN
OVER CURRENT
PROGRAMMABLE
OSCILLATOR
OVP
UVP
OCP
CLOCK
VSEN
OVP UVP
CONTROL LOGIC,
MONITOR, PROTECTIONS
&
EFFICIENCY OPTIMIZATION
+25%
ERROR AMPLIFIER
-25%
COMP
PGOOD
PWM
FB
CROSS CONDUCTION
ADAPTIVE ANTI
+
0.80V
-
FBG
VCC
2.2
HS
LS
L6738A
10k 10k
BOOT
UGATE
PHASE
VCCDR
LGATE
GND
8/32 Doc ID 18134 Rev 2
L6738A Pin description and connection diagrams
2 Pin description and connection diagrams
Figure 4. Pin connection (top view)
GND
VCC
OSC
CSP
16 15 14
VCCDR
LGATE
PHASE
BOOT
1
2
3
4
L6738A
6
UGATE
PGOOD
7 5
EN
2.1 Pin descriptions
Table 2. Pin description
Pin# Name Function
1 VCCDR
2LGATE
Low side driver section power supply.
Operative voltage is 5 V to 12 V bus. Filter with 1µF MLCC to GND.
Low-side driver output.
Connect directly to the low-side MOSFET gate. A small series resistor can be useful to
reduce dissipated power especially in high frequency applications.
13
12
11
10
9
8
COMP
CSN
FBG
VSEN
FB
High-side driver return path.
3 PHASE
4BOOT
5UGATE
6 PGOOD
7EN
Connect to the high-side MOSFET source. This pin is also monitored for the adaptive
dead-time management.
High-side driver supply.
This pin supplies the high-side floating driver. Connect through the C
PHASE pin. The pin is internally connected through a boot diode to the VCCDR pin. A
2.2Ohm series resistor is also provided. See Section 8 for guidance in designing the
capacitor value.
High-side driver output.
Connect to high-side MOSFET gate. A small series resistor may help in reducing the
PHASE pin negative spike as well as cooling the device.
Power Good.
It is an open-drain output set free after SS (with 3x clock cycle delay) as long as the out-
put voltage monitored through VSEN is within specifications. Pull-up to 3.3 V (typ) or
lower, if not used it can be left floating.
Enable Pin.
Pull-high to <5 V to enable conversion, 10 µA pull-down provided.
capacitor to the
BOOT
Doc ID 18134 Rev 2 9/32
Pin description and connection diagrams L6738A
Table 2. Pin description (continued)
Pin# Name Function
8COMP
9FB
Error amplifier output.
Connect with an RF - CF to FB. The device cannot be disabled by grounding this pin.
Error amplifier inverting input.
Connect with a resistor R
Output voltage monitor.
10 VSEN
It manages OVP and UVP protections and PGOOD. Connect to the positive side of the
load for remote sensing. See Section 6 for details.
11 FBG
12 CSN
13 CSP
Remote ground sense.
Connect to the negative side of the load for remote sensing.
Current sense negative input.
Connect to the output-side of the main inductor. Filter with 100 nF (Typ.) to GND.
Current sense positive input.
Connect through an R-C filter to the phase-side of the main inductor.
OSC: Internally set to 1.24 V, it allows programming the switching frequency F
14 OSC
device. Switching frequency can be increased according to the resistor R
to SGND with a gain of 10 kHz/µA (see Section 7 for details). If floating, the switching frequency is 200 kHz.
Device power supply. The embedded bootstrap diode is internally connected to this pin.
15 VCC
Operative voltage is 5 V to 12 V bus. Filter with 1 µF MLCC to GND.
For proper operations, VCC needs to be >1.5 V higher than the programmed V
16 GND
All internal references, logic and driver return path are referenced to this pin. Connect to
the PCB GND ground plane and filter to VCC and VCCDR.
to VSEN and with an RF - CF to COMP.
FB
OSC
of the
SW
connected
.
OUT
Thermal
PA D
The thermal pad connects the silicon substrate and makes good thermal contact with the
PCB. Use VIAs to connect to the PGND plane.
2.2 Thermal data
Table 3. Thermal data
Symbol Parameter Value Unit
R
R
T
T
THJA
THJC
MAX
STG
T
Thermal resistance junction to ambient
(Device soldered on 2s2p PC Board)
Thermal resistance junction to case 1 °C/W
Maximum junction temperature 150 °C
Storage temperature range -40 to 150 °C
Junction temperature range -40 to 125 °C
J
45 °C/W
10/32 Doc ID 18134 Rev 2
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