AN2815
Application note
High-efficiency step-down controller
with embedded 2 A LDO regulator
Introduction
The PM6675AS device consists of a single, high-efficiency step-down controller and an
independent low dropout (LDO) linear regulator.
The constant on-time (COT) architecture assures fast transient response supporting both
electrolytic and ceramic output capacitors. An embedded integrator control loop
compensates the DC voltage error due to the output ripple.
Selectable low-consumption mode allows the highest efficiency over a wide range of load
conditions. The low-noise mode sets the minimum switching frequency to 33 kHz for audio-
sensitive applications. The LDO linear regulator can sink and source up to 2 Apk. Two fixed
current limits (±1 A and ±2 A) can be chosen.
An active soft-end is independently performed on both the switching and the linear
regulators outputs when disabled.
Figure 1.
PM6675AS demonstration board
August 2008
Rev 1
1/42
www.st.com
Contents
AN2815
Contents
1
Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1
1.2
Switching section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
LDO section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2
3
4
5
6
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Component assembly and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
I/O interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Recommended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1
6.2
6.3
6.4
JP3 fixed or adjustable output voltage (VSEL pin) . . . . . . . . . . . . . . . . . . 14
JP1 power-saving mode (NOSKIP pin) . . . . . . . . . . . . . . . . . . . . . . . . . . 14
JP2 LDO current limit (LILIM pin) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
JP5 compensation network (COMP pin) . . . . . . . . . . . . . . . . . . . . . . . . . 16
7
8
9
Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
PM6675AS demonstration tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
V
OUT
and L
OUT
turn-on (soft-start) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
V
OUT
working mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
V
OUT
and L
OUT
load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
V
OUT
and L
OUT
load transient responses . . . . . . . . . . . . . . . . . . . . . . . . . 26
V
OUT
efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
V
OUT
gate drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
V
OUT
and L
OUT
turn-off (soft-end) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
UV, OV and thermal protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
V
OUT
current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
L
OUT
current limit (foldback) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2/42
List of figures
AN2815
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
PM6675AS demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
PM6675AS demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Top side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Layer 2 view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Layer 3 view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Bottom side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
JP3 (VSEL) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
JP1 (NOSKIP) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
JP2 (LILIM) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
JP5 (COMP) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
PM6675AS demonstration board test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
V
OUT
soft-start at 150 mW load, pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
L
OUT
turn-on, V
OUT
in pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
V
OUT
= 1.5 V, V
IN
= 24 V, I
VOUT
= 0 A, forced-PWM mode . . . . . . . . . . . . . . . . . . . . . . . . 21
V
OUT
= 1.5 V, V
IN
= 24 V, I
VOUT
= 0 A, pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . 22
V
OUT
= 1.5 V, V
IN
= 24 V, no load, non-audible pulse-skip mode (33 kHz) . . . . . . . . . . . . 23
V
OUT
load regulation - V
IN
= 24 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
L
OUT
load regulation - LDOIN = V
OUT
, V
OUT
in forced PWM mode . . . . . . . . . . . . . . . . . 25
V
OUT
load transient (V
IN
=24 V, LOAD=0 A to 7 A at 2.5 A/µs), pulse-skip mode. . . . . . . . 26
V
OUT
load transient (V
IN
= 24 V, LOAD=0 A to 7 A at 2.5 A/µs), pulse-skip mode. . . . . . . 27
Forced PWM (blue), non-audible pulse-skip (green), pulse-skip (red), efficiency vs.
output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
External MOSFET gate signals (V
IN
= 24 V, LOAD = 0), pulse-skip mode . . . . . . . . . . . . 29
External MOSFET gate signals (V
IN
= 24 V, LOAD = 7 A), pulse-skip mode . . . . . . . . . . 29
V
OUT
and L
OUT
output voltages, V
OUT
soft-end, L
OUT
powered by an auxiliary rail. . . . . . 30
V
OUT
and L
OUT
output voltages, L
OUT
soft-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
UV protection, pulse-skip mode, L
OUT
powered by an auxiliary rail . . . . . . . . . . . . . . . . . . 32
OV protection, pulse-skip mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
V
OUT
and L
OUT
rails, thermal shutdown, pulse-skip mode, L
OUT
powered by V
OUT
. . . . . 34
V
OUT
current limit protection during a load transient (0 A to 9 A at 2.5 A/µs . . . . . . . . . . . 35
L
OUT
current limit during an output short . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Switching frequency vs. input voltage, V
OUT
= 1.5 V, I
VOUT
= 2 A, forced PWM mode . . . 37
Forced PWM (blue), non-audible pulse-skip (green) and pulse-skip (red), Switching
frequency vs. output current, V
OUT
= 1.5 V, V
IN
= 24 V . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
I
LOUT
= 0 A, average IC temperature = 31.0 °C, max internal IC temperature = 33.1 °C. . 39
I
LOUT
= 0.2 A, average IC temperature = 38.2°C, max internal IC temperature = 40.7 °C . 39
I
LOUT
= 0.5 A, average IC temperature = 38.2 °C, max internal IC temperature = 41.5 °C 40
I
LOUT
= 1.0 A, average IC temperature = 48°C, max internal IC temperature = 55.2 °C . . 40
4/42
AN2815
Main features
1
1.1
Main features
Switching section
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4.5 V to 36 V input voltage range
0.6 V, ±1% voltage reference
1.5 V fixed output voltage
0.6 V to 3.3 V adjustable output voltage
1.237 V ±1% reference voltage available
Very fast load transient response constant on-time loop control
No-R
SENSE
current sensing using low-side MOSFETs' R
DS(on)
Negative current limit
Latched OVP, UVP and thermal shutdown
Fixed 3 ms soft-start
Selectable pulse-skipping at light load
Selectable non-audible (33 kHz) pulse-skip mode
All ceramic output capacitor applications supported
Output voltage ripple compensation
Output soft-end
1.2
LDO section
●
●
●
●
●
0.6 V to 3.3 V adjustable output voltage
Selectable ±1 A or ±2 A current limit
Dedicated Power Good signal
Ceramic output capacitors supported
Output soft-end
5/42
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